This page intentionally left blank A Clinical Guide to Inherited Metabolic Diseases
This user-friendly clinical handbook provides a clear and concise overview of how to go about recognizing and diagnosing inherited metabolic diseases. The reader is led through the diagnostic process from the identification of those features of an illness suggesting that it might be metabolic through the selection of appropriate laboratory investigation to a final diagnosis. The book is organized into chapters according to the most prominent presenting problem of patients with inherited metabolic diseases: neurologic, hepatic, cardiac, metabolic acidosis, dysmorphism, and acute catastrophic illness in the newborn. It also includes chapters on general principles, laboratory investigation, neonatal screening, and the principles of treatment. This new edition includes much greater depth on mitochondrial disease and congenital disor- ders of glycosylation. The chapters on neurological syndrome and newborn screening are greatly expanded, as are those on laboratory investigation and treatment, to take account of the very latest technological developments.
Dr J. T. R. Clarke is Professor of Paediatrics at the University of Toronto and a Senior Associate Scientist in the Research Institute of the Hospital for Sick Children, as well as continuing Head of the Genetic Metabolic Diseases Program of the Hospital. He is a Fellow of the Royal College of Physicians of Canada and the Canadian College of Medical Geneticists. He has won awards for postgraduate paediatric teaching and for excellence in paediatric medical care. He is consulted extensively by government and industry on matters relating to the management of inherited metabolic diseases and newborn screening. He is internationally respected for his expertise in the general area of metabolic genetics and is widely sought as a speaker and educator in this field. He has given over 100 invited lectures on inherited metabolic diseases in countries around the globe. This page intentionally left blank A Clinical Guide to Inherited Metabolic Diseases
Third Edition
JoeT.R.Clarke, MD, PhD, FRCPC Division of Clinical & Metabolic Genetics, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8 CANADA cambridge university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo
Cambridge University Press The Edinburgh Building, Cambridge cb2 2ru,UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Informationonthistitle:www.cambridge.org/9780521614993
© Joe T.R.Clarke 2005
This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press.
First published in print format 2005 isbn-13 978-0-511-13453-1 eBook (MyiLibrary) isbn-10 0-511-13453-3 eBook (MyiLibrary) isbn-13 978-0-521-61499-3 paperback isbn-10 0-521-61499-6 paperback
Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Contents
Reviewsoffirstedition page x Reviews ofsecond edition xi List of tables xii List of figures xvi Preface xix
1General principles 1 Introduction 1 Some general metabolic concepts 2 Disease results from point defects in metabolism 4 Accumulation of substrate 4 Accumulation of a normally minor metabolite 6 Deficiency of product 7 Secondary metabolic phenomena 9 Inborn errors of metabolism are inherited 9 Autosomal recessive disorders 9 X-linked recessive disorders 11 Autosomal dominant disorders 13 Mitochondrial inheritance 14 Inherited metabolic diseases may present at any age 16 Three sources of diagnostic confusion 17 Confusion with common acquired conditions 17 Confusion caused by association with intercurrent illness 20 Confusion arising from genetic heterogeneity 21 Congenital malformations and inborn errors of metabolism 21 The internet is particularly important 21 Suggested reading 27
2Neurologic syndrome 28 Chronic encephalopathy – without non-neural involvement 29 Gray matter disease (poliodystrophy) 29 Psychomotor retardation or dementia 29 Seizures 34 White matter disease (leukodystrophy) 47 v vi Contents
Chronic encephalopathy – with non-neural tissue involvement 50 Acute encephalopathy 53 Hyperammonemia 55 Leucine encephalopathy (maple syrup urine disease – MSUD) 61 Reye-like acute encephalopathy (fatty acid oxidation defects) 61 Acute encephalopathy with metabolic acidosis 63 Hypoglycemia 63 Stroke 63 Movement disorder 63 Ataxia 63 Choreoathetosis and dystonia 69 Parkinsonism 71 Myopathy 72 Acute intermittent muscle weakness 72 Progressive muscle weakness 73 Myoglobinuria (myophosphorylase deficiency phenotype) 74 Myoglobinuria (CPT II deficiency phenotype) 77 Myopathy as a manifestation of multisystem disease (mitochondrial myopathies) 77 Autonomic dysfunction 79 Psychiatric problems 79 Suggested reading 87
3Metabolic acidosis 90 Buffers, ventilation, and the kidney 90 Is the metabolic acidosis the result of abnormal losses of bicarbonate or accumulation of acid? 92 Metabolic acidosis caused by abnormal bicarbonate losses 92 Metabolic acidosis resulting from accumulation of organic anion 94 Lactic acidosis 94 Pyruvate accumulation 95 PDH deficiency 98 PC deficiency 99 Multiple carboxylase deficiency 99 NADH accumulation 100 Ketoacidosis 101 Mitochondrial acetoacetyl-CoA thiolase deficiency ( -ketothiolase deficiency) 102 Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency 103 Organic aciduria 104 Methylmalonic acidemia (MMA) 104 3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase deficiency 110 Glutaric aciduria 110 Dicarboxylic aciduria 112 Ethylmalonic aciduria 113 D-Lactic acidosis 113 vii Contents
Adventitious organic aciduria 114 Suggested reading 115
4Hepatic syndrome 116 Jaundice 116 Unconjugated hyperbilirubinemia 116 Conjugated hyperbilirubinemia 118 Hepatomegaly 118 Hypoglycemia 120 Ways to increase glucose production 120 Ways to decrease peripheral glucose utilization 122 An approach to the differential diagnosis of hypoglycemia 125 Hepatocellular dysfunction 133 Investigation 139 Liver function tests 139 Fasting tests 139 Suggested reading 141
5Cardiac syndromes 143 Cardiomyopathy 143 Initial investigation of possible inherited metabolic cardiomyopathy 148 Glycogen storage disease, type II (GSD II or Pompe disease) 149 Primary systemic carnitine deficiency 151 Fabry disease 151 Mitochondrial cardiomyopathies 152 Arrhythmias 153 Coronary artery disease 155 Familial hypercholesterolemia 155 Suggested reading 159
6Storage syndrome and dysmorphism 162 General characteristics of the dysmorphism resulting from inborn errors of metabolism 162 What are the types of inherited metabolic diseases in which dysmorphism might be expected to be prominent? 164 Lysosomal disorders 165 Peroxisomal disorders 178 Mitochondrial disorders 182 Biosynthetic defects 183 What sort of metabolic studies are most likely to be diagnostically productive in the investigation of dysmorphism? 195 Suggested reading 196
7Acute metabolic illness in the newborn 198 Suspicion 198 Initial laboratory investigation 200 Five clinical ‘syndromes’ 202 viii Contents
Encephalopathy without metabolic acidosis 203 Maple syrup urine disease (MSUD) 203 Urea cycle enzyme defects (UCED) 205 Nonketotic hyperglycinemia (NKHG) 208 Pyridoxine-dependent seizures 209 Peroxisomal disorders (Zellweger syndrome) 209 Molybdenum cofactor deficiency (sulfite oxidase/xanthine oxidase deficiency 209 Encephalopathy with metabolic acidosis 210 Organic acidurias 210 Congenital lactic acidosis 213 Dicarboxylic aciduria 214 Neonatal hepatic syndrome 215 Jaundice 215 Severe hepatocellular dysfunction 217 Hypoglycemia 220 Cardiac syndrome 222 Intractable cardiac arrhythmias 222 Cardiomyopathy 222 Nonimmune fetal hydrops 223 Initial management 223 Summary comments 226 Suggested reading 226
8Newborn screening 228 Screening for medical intervention 230 Screening for reproductive planning 230 Screening to answer epidemiological questions 231 Case-finding 232 Problems created by false positive screening tests 232 Screening technology 233 Bacterial inhibition assays – the ‘Guthrie test’ 233 Tandem MSMS 235 Radioimmunoassay 238 Enzyme assay 238 Specific mutation testing 239 Suggested reading 239
9 Laboratory investigation 241 Studies on the extent and severity of pathology 245 Studies directed at the classification of disease processes (the ‘metabolic screen’) 245 Investigation of ‘small molecule disease’ 246 Plasma ammonium 246 Plasma lactate and pyruvate 246 Plasma ketones and free fatty acids 246 Amino acid analysis 247 Neurotransmitters 253 Organic acid analysis 254 Acylcarnitines and acylglycines 256 Porphyrins 260 Approaches to the investigation of metabolic disorders 263 ix Contents
Cellular metabolic screening studies 263 Provocative testing 267 Enzymology 270 Molecular genetic studies 271 Investigation of ‘organelle disease’ 274 Lysosomal disorders 274 Disorders of mitochondrial energy metabolism 285 Peroxisomal disorders 291 Suggested reading 295
10 Treatment 297 Control of accumulation of substrate 297 Restricted dietary intake 297 Control of endogenous production of substrate 301 Acceleration of removal of substrate 303 Replacement of product 306 Reaction product replacement 306 Gene product replacement 307 Gene product stabilization 309 Cofactor replacement therapy 309 Mitochondrial electron transport defects 313 Gene transfer therapy 314 Organ transplantation 314 Single gene transfer therapy 317 Supportive measures 318 Special considerations for adults with inherited metabolic diseases 318 Suggested reading 321
Index 324 Reviews of first edition
‘should be read thoroughly by any pediatricresident, genetic resident, or clinical fellow caring for patients with metabolic disorders’ American Journal of Medical Genetics ‘In short, this is an excellent guide to metabolic disease; it represents good value for money and, I suspect, will be more likelyfound in the owner’s pocket rather than on the shelf. It is recommended not only to the ‘busy physician’ and trainee, but to all those with an interest in metabolic disease.’ Journal of Inherited Metabolic Disease ‘The writing is lucid, direct and salted with personal observations. Clarke’s teaching skills shine forth from each page . . . It succeeds admirably, effectively demystifying the anxiety-provoking world of inherited biochemical illness.’ Canadian Medical Association Journal ‘J. T. R. Clarke has performed the amazing feat of distilling practical knowledge about the diagnosis of metabolic diseases into a small, yet ultimately pragmatic 280-page clinical guide . . . On the whole, I found this to be an amazing book which contains a vast amount of information presented in a concise, logical and well- organized fashion . . . I would recommend this book wholeheartedly to anyone involved in the diagnosis of inherited metabolic diseases.’ Journal of Genetic Counseling
x Reviews of second edition
‘Dr Clarke’s enthusiasm and erudition are evident on every page of this book.’ Archives of Diseases of Childhood ‘Anexcellent book for physicians who find inherited metabolic diseases intimidat- ing...Theinformationispresentedinsuchaclear and simple fashion that few people would find this book difficult to read . . . Clarke teaches a complex subject in a simple but complete manner.’ Canadian Medical Association Journal ‘This book’s strength lies in its simple straightforward clinical approach to this difficult area of medicine.’ Doctors.net.uk ‘If your clinical work brings you into contact with patients who may be hiding an inherited metabolic disease, Clarke’s Guide is clearly for you.’ Journal of the Royal Society of Medicine ‘To guide the reader in this assessment, a compact volume such as has been written by Dr Clarke is invaluable. Dr Clarke has succeeded in providing the reader with a user-friendly, inexpensive book that is up to date, and provides directions for further reading.’ European Journal of Paediatric Neurology
xi List of tables
1.1. Some examples of inborn errors of metabolism in which symptoms of disease are the result of substrate accumulation. page 5 1.2. Some examples of inborn errors of metabolism in which symptoms of disease are the result of product deficiency. 8 1.3. Some examples of inborn errors of metabolism in which secondary metabolic defects play a prominent role in the production of symptoms of disease. 10 1.4. Some examples of inborn errors of metabolism occurring with high frequency among specific ethnic groups. 12 1.5. Some mechanisms of autosomal dominance. 13 1.6. IEM presenting in adulthood. 18 1.7. Some common non-metabolic conditions that are often confused with inherited metabolic diseases. 20 1.8. Disease websites. 22 1.9. Mutation websites. 24 2.1. Causes of pseudo-regression. 32 2.2. Initial investigation. 34 2.3. Seizures. 36 2.4. Classification of NCL. 40 2.5. Causes of acute metabolic encephalopathy to be considered at various ages. 54 2.6. Initial investigation of acute encephalopathy. 55 2.7. Differential diagnosis of inherited metabolic diseases presenting as acute encephalopathy. 55 2.8. Urinary organic acid abnormalities in the hereditary fatty acid oxidation defects. 64 2.9. Inherited metabolic diseases in which stroke may be a prominent feature. 65 xii xiii List of tables
2.10. Inherited metabolic diseases in which extrapyramidal movement disorders are prominent. 66 2.11. Protocol for the ischemic forearm exercise test. 75 2.12. Inherited metabolic diseases presenting as muscle cramping or myoglobinuria. 76 2.13. Differences between myophosphorylase deficiency and CPT II deficiency phenotypes. 77 2.14. Some common features of conditions caused by mitochondrial mutations. 79 2.15. Main clinical features of some relatively common mitochondrial syndromes. 80 2.16. Some mitochondrial syndromes caused by nDNA mutations. 81 2.17. Autonomic dysfunction 84 2.18. Some inherited metabolic diseases characterized by psychiatric or severe behavioral abnormalities. 85 3.1. Inherited metabolic diseases associated with renal tubular acidosis (RTA). 93 3.2. Clinical classification of lactic acidosis. 96 3.3. Urinary organic acids in multiple carboxylase deficiency. 100 3.4. Organic acidurias. 105 3.5. Flavoprotein dehydrogenases for which ETF/ETF dehydrogenase is the electron acceptor. 111 3.6. Some common causes of spurious or artefactual organic aciduria. 114 4.1. Causes of secondary carnitine deficiency. 124 4.2. Approach to hypoketotic hypoglycemia. 131 4.3. Relationship between metabolic defects and clinical manifestations of mitochondrial fatty acid -oxidation defects. 133 4.4. Inherited metabolic diseases presenting as severe hepatocellular dysfunction organized according to age of onset. 134 4.5. Investigation of liver function. 139 5.1. Inherited metabolic diseases in which cardiomyopathy is prominent. 144 5.2. mtDNA point mutations associated with cardiomyopathy. 153 5.3. Some general clinical characteristics of cardiomyopathies 154 5.4. Plasma lipoproteins. 156 5.5. Familial hyperlipidemias. 158 6.1. Classification of inborn errors with significant dysmorphism. 164 6.2. Some clinical features of lysosomal disorders presenting as ‘storage syndrome’. 171 6.3. Classification of peroxisomal disorders. 181 6.4. Conditions associated with chondrodysplasia punctata. 182 xiv List of tables
6.5. IEM of sterol biosynthesis. 187 6.6. Classification of subtypes of congenital disorders of glycosylation (CDG) syndrome. 188 6.7. Genetic and some acquired causes of homocystinuria. 193 6.8. Initial metabolic investigation of patients presenting with storage syndrome or dysmorphism. 195 7.1. Suspicion of an inherited metabolic disease. 199 7.2. Initial laboratory investigation ofsuspected inherited metabolic disease presenting in the newborn period. 201 7.3. Inherited metabolic disorders presenting in the newborn period with acute encephalopathy without metabolic acidosis. 203 7.4. Organic acidopathies presenting as acute illness in the newborn period. 211 7.5. Inherited metabolic diseases presenting with severe hepatocellular dysfunction in the newborn period. 218 7.6. Inherited metabolic diseases presenting as nonimmune fetal hydrops. 224 7.7. Initial management of possible inherited metabolic diseases presenting in the newborn period. 225 8.1. Classification of inherited disorders of phenylalanine metabolism. 234 8.2. Investigation of neonatal hyperphenylalaninemia. 235 8.3. Disorders detectable by tandem MSMS. 237 9.1. Clinical differentiation of organelle disease and small molecule disease. 243 9.2. Some general clinical characteristics of defects of organelle metabolism. 244 9.3. Chemical screening tests for amino acids and amino acid metabolites in urine. 249 9.4. Plasma amino acid abnormalities in various primary disorders of amino acid metabolism. 251 9.5. Primary disorders of amino acid transport. 253 9.6. Some common secondary abnormalities of plasma or urinary amino acids. 254 9.7. Neurotransmitter abnormalities 255 9.8. Biochemical abnormalities in the porphyrias. 260 9.9. Secondary porphyrias 263 9.10. Some common mutations causing specific inherited metabolic diseases. 272 9.11. Urinary MPS in the different mucopolysaccharidoses (MPS). 281 9.12. Summary of assays useful in the investigation of lysosomal storage diseases. 284 9.13. Subunits of mitochondrial electron transport chain. 290 9.14. Mitochondrial mutation analysis. 292 9.15. Some key functions of peroxisomes. 294 9.16. Laboratory abnormalities in some of the peroxisomal disorders. 294 xv List of tables
10.1. Some examples of inborn errors of metabolism treatable by dietary manipulation. 299 10.2. Various cofactors involved in intermediary metabolism and implicated in some cofactor-responsive inborn errors of metabolism. 310 10.3. Treatment by organ transplantation. 315 List of figures
1.1. The primary consequences of inborn errors of metabolism. page 4 1.2. The human mitochondrial genome. 15 1.3. The effect of heteroplasmy on the clinical expression of mtDNA mutations. 16 2.1. An approach to chronic encephalopathy. 33 2.2. MRI scans of the brain in patient with juvenile neuronal ceroid-lipofuscinosis. 38 2.3. Electron micrograph of conjunctival epithelium showing curvilinear and fingerprint inclusions in a patient with neuronal ceroid-lipofuscinosis. 39 2.4. Axial MRI scan and MRS of brain of a patient with Leigh disease. 42 2.5. Coronal MRI scan of the brain of a child with Leigh disease. 43 2.6. Axial MRI scan of the brain of a child with MELAS. 44 2.7. Axial CT scan of the brain of an infant with Tay-Sachs disease. 46 2.8. CT scan of the brain of an infant with Canavan disease. 47 2.9. CT and MRI scans of the brain in X-linked adrenoleukodystrophy. 49 2.10. MRI scan of the brain in late-onset GM2 gangliosidosis. 50 2.11. Summary of the major causes of acute encephalopathy. 54 2.12. Summary of normal ammonium metabolism. 57 2.13. An approach to the diagnosis of hyperammonemia in older children. 58 2.14. MRI in GAI. 70 2.15. Photomicrograph of skeletal muscle stained by the modified Gomori trichrome method showing ragged-red fibers. 83 3.1. Approach to the investigation of metabolic acidosis. 95 3.2. Metabolic sources and fates of pyruvate. 97 3.3. Summary of ketone metabolism. 102 3.4. Branched-chain amino acid metabolism. 103 3.5. Relationship between cobalamin, methylmalonic acid (MMA), and homocysteine metabolism. 108 xvi xvii List of figures
4.1. Overview of key reactions in gluconeogenesis. 121 4.2. The Coricycle. 123 4.3. Approach to the differential diagnosis of hypoglycemia. 126 4.4. Electron micrograph of normal liver (a) and liver in glycogen storage disease (b). 128 4.5. Overview of fatty acid metabolism. 132 4.6. Electron micrograph of liver in Wilson disease. 138 5.1. Investigation of metabolic cardiomyopathy. 148 5.2. Radiograph of the chest in an infant with Pompe disease (GSD II). 150 5.3. ECG of infant with Pompe disease (GSD II). 150 5.4. Typical angiokeratomata on the penis of a man with Fabry disease. 152 5.5. Tuberous xanthomas on the hands of a seven-year-old boy with familial hypercholesterolemia. 157 6.1. Facial features of patients with various lysosomal storage disorders. 166 6.2. Radiographs of patient with Hurler disease (MPS IH). 169 6.3. Alder-Reilly bodies in peripheral blood lymphocyte from patient with Hurler disease (MPS IH). 170 6.4. Radiograph and MRI scan of the lower extremities in Gaucher disease. 175 6.5. Typical storage macrophage in the bone marrow of a patient with Gaucher disease. 176 6.6. Facial features of a newborn infant with classical Zellweger syndrome. 178 6.7. Radiograph of the knee in a patient with Zellweger syndrome showing stippling of the patella. 179 6.8. MRI scan of the brain in patient with Zellweger syndrome. 180 6.9. Facial features of an infant with Menkes disease. 184 6.10. Facial features of an infant with mevalonic aciduria. 185 6.11. Smith-Lemli-Opitz syndrome. 186 6.12. Lipodystrophy of the buttocks and thighs of a 6-week-old infant with congenital disorder of glycosylation syndrome. 190 6.13. Isoelectric focusing of plasma transferrin in CDG syndrome. 192 6.14. Metabolism of homocysteine and methionine. 193 7.1. Imaging of the brain of an infant with maple syrup urine disease. 204 7.2. Differential diagnosis of urea cycle enzyme defects in the newborn. 206 9.1. Some typical pathological urinary organic acid patterns. 257 9.2. Heme biosynthesis and the porphyrias. 262 9.3. Schematic diagram showing principles of complementation analysis. 265 9.4. Principles of cybrid analysis. 267 9.5. Blue native polyacrylamide gel electrophoresis (BN-PAGE) analysis. 268 9.6. Electron micrograph of conjunctival biopsy in mucolipidosis type IV. 276 xviii List of figures
9.7. Electron micrographs of skin biopsies in some lysosomal storage diseases. 277 9.8. Summary of glycoprotein degradation. 282 9.9. Thin-layer chromatographic analysis of urinary oligosaccharides. 283 9.10. Overview of mitochondrial fatty acid metabolism. 287 9.11. Mitochondrial fatty acid oxidation. 288 9.12. Complexes of the mitochondrial electron transport chain. 289 10.1. Effects of NTBC (2-(2-nitro-4-trifluoromethylbenoyl) -1,3-cyclohexane-dione) on tyrosine metabolism. 303 Preface
In this enlarged third edition of A Clinical Guide to Inherited Metabolic Diseases, Ihavepreserved the basic, clinical approach first developed in the first edition of the book. However, advances in many fields over the past 5 years have made it necessary to add significantly in some areas, such as mitochondrial disorders and the congenital disorders of glycosylation. The challenge continues to be to find ways to translate discoveries made in research laboratories, which understandably focus on biochemical and genetic principles, into a clinically relevant format organized in a way to facilitate the early recognition of the disorders by clinicians. For exam- ple, inherited defects in mitochondrial electron transport (ETC) may present as neurological syndromes (encephalopathy, myopathy, movement disorder), cardiac syndrome(cardiomyopathy),hepaticsyndrome,metabolicacidosis,orcatastrophic illness in the newborn. The challenge has been to develop and present a clinical approach to mitochondrial ETC defects without being unnecessarily repetitious. The chapter on ‘Laboratory investigation’ is important in this respect because it provides an approach to the transition in thinking between the recognition of var- ious clinical signs and the biochemical and genetic investigation of possible causes of disease. By the very nature of laboratory investigation, it is also organized bio- chemically, which draws together the consideration of all those disorders presented in various different chapters as clinical problems. The book should, therefore, be viewed as a series of clinical chapters, which overlap in terms of biochemical and genetic organization and content in the chapteron‘Laboratory investigation’. It follows that reference to any topic presented in a clinical chapter ought to be con- sidered also in the light of the appropriate section on the chapter on ‘Laboratory investigation’ – they go together. Ihaveadded significantly to the chapter, ‘Neurologic syndrome’, as new mech- anisms of disease are discovered, such as inherited disorders of neurotransmitter metabolism and the channelopathies. The chapter on ‘Newborn screening’ has also been expanded as this field grows, along with experience with the application of tandem mass spectrometry as a screening technology. The chapters on ‘Laboratory xix xx Preface
investigation’ and ‘Treatment’ have had to be modified significantly in the light of new technological developments in these areas. I have also added to and updated the bibliographies at the end of each chapter, though the number of publications cited is still a tiny fraction of the literature on the subjects discussed. In many cases, Ihavesacrificed some outstanding articles focusing on advances in basic research for articles I thought would be more relevant to clinicians dealing daily with patient problems. For intellectual support and stimulation during the preparation of this edition of A Clinical Guide,Iamagain grateful to my colleagues, Annette Feigenbaum, Susan Blaser, Bill Hanley, Brian Robinson, John Callahan, and Eve Roberts, and to the people who slave away in the diagnostic labs, all at the Hospital for Sick Children. In addition, however, I owe a great deal to colleagues in other centers, scattered throughout the world, who read the second edition and suggested some changes which I am convinced will make this edition even better. I owe Charles Scriver special thanks for comments on the last edition of this book which have resulted in some important additions to the current edition. As usual, I am indebted to the large number of residents and fellows who rotated through the genetic metabolic service at the Hospital, stimulating me to think clearly about the clinical problems we tackled together. Gustavo Maegawa, from Brazil, Nouriya Al-Sannaa, now in Dhahran, Saudi Arabia, Aneal Khan, now at McMaster University in Hamilton, Pranesh Chakraborty, who is now at the Children’s Hospital of Eastern Ontario, Nicola Poplawski, at the Adelaide Women’s and Children’s Hospital in Adelaide, and Julian Raiman, now at Guy’s Hospital in London, merit special mention in this regard. Many colleagues provided material for the figures in the book: Jim Phillips pro- vided the electron micrographs of the liver, and Venita Jay supplied the electron micrographs of conjunctival epithelium and the photomicrograph of muscle. The photographs of patients with carbohydrate-deficient glycoprotein syndrome (now called congenital disorders of glycosylation) and mevalonic aciduria were provided by Jaak Jaiken and Georg Hoffmann, respectively. Jaak Jaiken also supplied the photograph of the isoelectric focusing of plasma transferrin shown in Chapter 6. JoeAlroy kindly provided the original electron micrographs showing the changes in skin in patients with lysosomal disorders appearing in Chapter 9. Margaret Nowaczyk and Chitra Prasad provided photographs of patients with Smith-Lemli- Opitz syndrome (Chapter 6), and Eric Shoubridge provided the photograph of the blue native PAGE in Chapter 9. I am again particularly grateful to Susan Blaser for the neuroimaging studies reproduced in Chapter 2 on ‘Neurologic syndrome’. PeterSilver, at Cambridge University Press, continued to provide moral and technical support during the preparation of the book. And once again, my wife, Cathy,encouragedandsupportedmethroughouttheproject,oftenatgreatpersonal cost. 1 1 General principles
Introduction
In his 1908 address to the Royal College of Physicians of London, Sir Archibald Garrod (1857–1936) coined the expression inborn error of metabolism to describe agroup of disorders – alkaptonuria, benign pentosuria, albinism, and cystinuria – which “. . . apparently result from failure of some step or other in the series of chemical changes which constitute metabolism”.1 He noted that each was present at birth, persisted throughout life, was relatively benign and not significantly affected by treatment; and that each was transmitted as a recessive trait within families in a way predictable by Mendel’s laws of inheritance. In fact, alkaptonuria was the first example of Mendelian recessive inheritance to be recognized as such in humans. Garrod concluded, from the results of experiments on the effects of feeding phenylalanine and various putative intermediates of phenylalanine metabolism on homogentisic acid excretion by patients with the condition, that homogentisic acid is an intermediate in the normal metabolism of phenylalanine and tyrosine. Moreover, he observed that the specific defect was in the oxidation of homogentisic acid. La Du subsequently confirmed this 50 years later by demonstrating profound deficiency of homogentisic acid oxidase in a biopsy specimen of liver from a patient with alkaptonuria.2 Following Følling’s discovery of phenylketonuria (PKU) in 1934, Garrod’s con- cept underwent a major change, particularly with respect to its relationship with disease. Like alkaptonuria, PKU was shown to be caused by a recessively inherited point defect in metabolism, in the conversion of phenylalanine to tyrosine in the liver. However, unlike Garrod’s original four inborn errors of metabolism, PKU was far from benign – it was associated with a particularly severe form of mental retardation. Moreover, although the underlying metabolic defect was ‘inborn’ and
1 H. Harris, Garrod’s inborn errors of metabolism,London: Oxford University Press, 1963, p. 13. 2 La Du, B. N., Zannoni, V. G., Laster, L. & Seegmiller, J. E. (1958). The nature of the defect in tyrosine metabolism in alcaptonuria. Journal of Biological Chemistry, 230, 251–60.
1 2 A Clinical Guide to Inherited Metabolic Diseases
persisted throughout life, the associated mental retardation could be prevented by treatment with dietary phenylalanine restriction. HarryHarris (1919–1994) applied the technique of starch gel electrophoresis, described originally by Smithies in 1955, to the demonstration of a large number of protein polymorphisms in humans, confirming the vast biochemical genetic diversity predicted by Garrod. He went on to show how the principle of one gene- one polypeptide chain applied to Garrod’s inborn errors of metabolism, which he reviewed in some detail in his highly successful book, The Principles of Human Biochemical Genetics3, published originally in 1970. The discovery of PKU sparked the search for other clinically significant inborn errors of metabolism. The most recent edition of The Metabolic and Molecular Bases of Inherited Disease4,edited by Charles Scriver and his colleagues, reports that the number of diseases in humans known to be attributable to inherited point defects in metabolism now exceeds 500. While the diseases are individually rare, they collectively account for a significant proportion of illness, particularly in children. They present clinically in a wide variety of ways, involving virtually any organ or tissue of the body, and accurate diagnosis is important both for treatment and for the prevention of disease in other family members. While inborn errors of metabolism are a well-known cause of disease in chil- dren, their contribution to disease in adults is less well appreciated. This may be due in part to the lethal nature of many of the diseases, which often cause death before the patient reaches adulthood. However, owing primarily to advances in diagnostic technology, especially molecular genetics, a rapidly growing list of inherited metabolic diseases presenting in adulthood is emerging. Some of these, such as Scheie disease (MPS IS), are milder variants of diseases caused by the same enzyme deficiency that commonly causes death in childhood. Others, such as clas- sical hemochromatosis, almost never present in childhood, though the disease in adults may be severe. The purpose of this book is to provide a framework of principles to help clinicians recognize when an illness might be caused by an inborn error of metabolism. It presents a problem-oriented clinical approach to determining the type of metabolic defect involved and what investigation is needed to establish a specific diagnosis.
Some general metabolic concepts
Metabolism is the sum total of all the chemical reactions constituting the continu- ing process of breakdown and renewal of the tissues of the body. Enzymes play an
3 H. Harris, The Principles of Human Biochemical Genetics, Amsterdam: North-Holland Publishing, 1970. 4 Scriver, C. R., Beaudet, A. L., Sly, W. S. & Valle D. (eds). (2001). The Metabolic and Molecular Bases of Inherited Disease,8th ed. New York: McGraw-Hill. 3 General principles
indispensable role in facilitating the process by serving as catalysts in the conversion of one chemical (metabolite) to another, often extracting the energy required for the reaction from a suitable high-energy source, such as ATP. All enzymes have at least two types of physico-chemical domains: one or more substrate-binding domains, and at least one catalytic domain. Mutations might affect enzyme activ- ity by affecting the steady-state amount of enzyme protein because of a defect in enzyme production or as a result of abnormally rapid breakdown of the mutant protein. Deletions and insertions causing shifts in the reading frame, mutations to premature stop codons, and mutations affecting the processing of the pri- mary RNA transcript all affect enzyme activity by decreasing the production of active enzyme protein. In contrast, studies on the turnover of mutant enzyme proteins have shown that many single amino acid substitutions resulting from single base substitutions cause deficiency of enzyme activity by causing abnor- mal folding of the nascent enzyme polypeptide which in turn causes aggregation and premature destruction of the polypeptide before it leaves the endoplasmic reticulum. This discovery is of signal importance to the development of new and specific treatments for diseases caused by inborn errors of metabolism because the stability of many mutant enzyme proteins, and therefore the activity of the enzymes, is enhanced by exposure to ‘chemical chaperones’. These are relatively simple and inexpensive chemicals, in some cases substrate analogues, that bind to the mutant enzyme protein, preventing premature degradation. Mutations might also impair the activity of the enzyme without affecting the amount of enzyme protein by specifically impairing the catalytic properties of the protein. Of all the possible mechanisms of enzyme deficiency caused by mutation, this is the most uncommon. The rapid transport of metabolites across cellular and subcellular membranes is facilitated in many cases by specific transport proteins that function like enzymes. This means that the process is susceptible to genetic mutations affecting the amount or function of the transporter in exactly the same way that mutation affects the activities of enzymes, and with similar consequences. The coding sequences of most structural genes are comprised of at least a few thousand nucleotides, and the potential for mutation-generated variations in nucleotide sequence is vast. In the same way, the effects of mutation also vary tremendously. At one extreme, some mutations may totally disrupt the production of any gene product, resulting in severe disease. By contrast, other mutations might have no effect whatsoever apart from a functionally silent change in the nucleotide sequence of the gene. The relationship between genotype and disease phenotype is complex. Severe mutations, such as deletions or insertions, are generally asso- ciated with clinically severe disease, and the disease phenotype among different affected individuals tends to be similar. Structurally more subtle mutations, such 4 A Clinical Guide to Inherited Metabolic Diseases
Figure 1.1 The primary consequences of inborn errors of metabolism. The figure shows diagrammatically the various possible mutation-sensitive defects affect- ing the compartmentalization and metabolism of Compound A. 1, transporter-mediated movement of A from one compartment to another; 2, defect in the conversion of B to C; 3, increased conversion of B to D caused by accumulation of B; 4, defect in the interaction between an apoenzyme and an obligatory cofactor; 5, decreased feedback inhibition of
the conversion of Ain to B as a result of deficiency of C; and 6, secondary inhibition of the conversion of E to F caused by accumulation of D.
as those resulting in single amino acid substitutions, are often associated with milder disease phenotypes. Moreover, the disease phenotype often varies markedly between different affected individuals, even within the same family, a reminder that the expression of any genetic information, including disease-causing mutations, is influenced by other genes (gene-gene interactions) and by environmental factors (gene-environment interactions).
Disease results from point defects in metabolism
The signs and symptoms of disease in patients with inborn errors of metabolism are the result of metabolic disturbances caused by deficiency of some catalytic or transport protein. Figure 1.1 shows schematically the relationship between vari- ous types of defects and their pathophysiologically and diagnostically important consequences.
Accumulation of substrate Accumulation of the substrate of a mutant enzyme is an important cause of disease in many inborn errors of metabolism, particularly those involving strictly degrada- tive processes. Some examples are shown in Table 1.1. Table 1.1 Some examples of inborn errors of metabolism in which symptoms of disease are the result of substrate accumulation
Disease Metabolic defect Accumulating substrate Main clinical findings
Tay-Sachs disease -Hexosaminidase A deficiency GM2 ganglioside Cerebral neurodegeneration OTCdeficiency OTC deficiency Ammonium Acute encephalopathy Methylmalonic acidemia Methylmalonyl-CoA mutase deficiency Methylmalonic acid Metabolic acidosis PKU Phenylalanine hydroxylase deficiency Phenylalanine Progressive mental retardation Hurler disease -l-iduronidase deficiency Dermatan and heparan sulfates Unusual facies, skeletal abnormalities, progressive mental retardation Cystinuria Dibasic amino acid transport defect in Cystine in urine Recurrent obstructive uropathy kidney Hepatorenal tyrosinemia Fumarylacetoacetase deficiency Fumarylacetoacetate and Acute hepatocellular dysfunction, cirrhosis, rickets maleylacetoacetate
Abbreviations:OTC,ornithine transcarbamoylase; PKU, phenylketonuria. 6 A Clinical Guide to Inherited Metabolic Diseases
Accumulation of substrate is also diagnostically important. Specific diagnosis often follows quickly after identificationofthe accumulation of metabolites prox- imal to an enzyme defect, particularly among inborn errors of water-soluble sub- strates. This is generally true, for example, of the amino acidopathies and organic acidopathies, in which accumulation of substrate throughout the body is often massive, and is reflected by changes in plasma and urine. In inborn errors of metabolism involving water-insoluble substrates, such as complex lipids, accumulation of the immediate substrate of the mutant enzyme is also important in the pathophysiology of disease. However, accumulation of the compounds is often limited to single tissues or organs, such as brain, which are relatively inaccessible. Moreover, chemical isolation and identification of the metabolites is often cumbersome, requiring laboratory expertise that is not rou- tinely available. In other disorders, such as the mucopolysaccharide storage diseases, the accu- mulation of substrate is a major factor in the pathophysiology of disease. However, because the metabolism of the substrate requires the participation of a number of different enzymes, any of which may be deficient as a result of mutation, the demonstration of accumulation is diagnostically important only to the extent that it indicates a class of disorders, not one specific disease. The demonstration of mucopolysaccharide accumulation is important as a screening test for inherited defects in mucopolysaccharide metabolism. However, the metabolism of the indi- vidual mucopolysaccharides involves 10 or more genetically distinct lysosomal enzymes, and accumulation of the same compound may occur as a conse- quence of deficiency of any one of the enzymes. Specific diagnosis in these dis- orders requires the demonstration of the specific enzyme deficiency in appro- priate tissues, such as peripheral blood leukocytes or cultured skin fibroblasts (see Chapter 9).
Accumulation of a normally minor metabolite In some disorders, the primary cause of disease is accumulation of a normally minor metabolite, produced in excess by a reaction that is usually of trivial metabolic importance. The cataracts in patients with untreated galactosemia occur as a result of accumulation the sugar alcohol, galactitol, a normally minor metabolite of galactose. In another example, accumulation of the normally minor complex lipid metabolite, psychosine, in the brain of infants with Krabbe globoid cell leukodys- trophy excites a subacute inflammatory reaction, manifested by appearance in the brain of multinucleated giant cells, called globoid cells. It also causes rapid, severe demyelination, out of proportion to the accumulation of galactocerebroside, the immediate precursor of the defective enzyme, galactocerebrosidase. 7 General principles
Deficiency of product Deficiency of the product of a specific reaction is another primary consequence of many inherited metabolic diseases. The extent to which it contributes to disease depends on the importance of the product. For example, most of the pathologic consequences of defects of biosynthesis are traceable to deficiency of the product of the relevant reaction – in these cases substrate accumulation plays little or no role in the development of disease. Table 1.2 shows a list of some conditions in which symptoms are the result of deficiency of the product of some enzymic reaction or transport process. Among the inborn errors of amino acid biosynthesis, the signs of disease are often the combined result of substrate accumulation and product deficiency. For example, in the urea cycle disorder, argininosuccinic aciduria, the defect in the conversion of argininosuccinic acid to arginine causes arginine deficiency, and this, in turn, results in a deficiency of ornithine. Depletion of intramitochondrial ornithine causes accumulation of carbamoylphosphate and ammonia resulting in marked hyperammonemic encephalopathy. The importance of arginine defi- ciency in the pathophysiology of the encephalopathy is shown by the dramatic response to administration of a single large dose of arginine (4 mmoles/kg given intravenously). Deficiency of products of reactions is important in two other situations that are common among the inborn errors of metabolism. One of these could be regarded as the result of a ‘metabolic steal’,a term used to explain the occurrence of myopathy in some patients with glycogen storage disease due to debrancher enzyme deficiency. It was postulated that increased gluconeogenesis in patients with the disease causes accelerated muscle protein breakdown as free amino acids are diverted from pro- tein biosynthesis to gluconeogenesis in an effort to maintain the blood glucose in the face of impaired glycogen breakdown. Another example of the consequences of a metabolic steal is the occurrence of hypoglycemia in patients with heredi- tary defects in fatty acid oxidation. The over-utilization of glucose and resulting hypoglycemia are a consequence of the inability to meet energy requirements by fatty acid oxidation because of deficiency of one of the enzymes involved in the process. Another mechanism by which a metabolic defect causes symptoms because of deficiency or inaccessibility of a product might be called ‘metabolic sequestration’. Transport defects caused by mutations affecting proteins involved in carrier- mediated transport often produce disease through a failure of the transfer of a meta- bolite from one subcellular compartment to another. The HHH syndrome, named for the associated hyperammonemia, hyperornithinemia, and homocitrullinemia, is caused by a defect in the transport of the amino acid, ornithine, into the Table 1.2 Some examples of inborn errors of metabolism in which symptoms of disease are the result of product deficiency
Disease Metabolic defect Product deficiency Main clinical findings
Vitamin D dependency 25-Hydroxycholecalciferol-1 - 1 , 25-Dihydroxycholecalciferol Rickets hydroxylase deficiency Hartnup disease Neutral amino acid transport defect Niacinamide Pellagra-like condition Lysinuric protein intolerance Dibasic amino acid transport defect Ornithine Recurrent hyperammonemia Hereditary thrombophilia Protein C defect Protein C (physiologic anticoagulant) Recurrent phlebothrombosis
Transcobalamin II deficiency Transcobalamin II defect Vitamin B12 Megaloblastic anemia Congenital hypothyroidism Various defects in thyroid hormone Thyroid hormone Cretinism, goitre biosynthesis X-linked hypophosphatemic rickets Renal phosphate transport defect Phosphate Rickets 9 General principles
mitochondria. The resulting intramitochondrial ornithine deficiency causes accu- mulation of carbamoylphosphate and ammonia, ultimately causing hyperam- monemic encephalopathy, in a manner similar to that causing the hyperammone- mia in argininosuccinic aciduria described above.
Secondary metabolic phenomena Because of the close relationship between the various processes comprising interme- diary metabolism, enzyme deficiencies or transport defects inevitably have effects beyond the immediate changes in the concentrations of substrate and product of any particular reaction. These secondary metabolic phenomena often cause diag- nostic confusion. For example, ketotic hyperglycinemia was initially thought to be a primary disorder of glycine metabolism. However, subsequent studies showed that glycine accumulation was actually a secondary metabolic phenomenon in patients with a primary defect of propionic acid metabolism. Furthermore, the acute forms ofotherorganicacidopathies,suchasmethylmalonicacidemia(seeChapter3),were also found to be associated with marked accumulation of glycine, severe ketoacido- sis, and hyperammonemia, all the result of secondary metabolic effects of organic acid or organic acyl-CoA accumulation. Table 1.3 lists some examples of potentially confusing secondary metabolic responses to point defects in metabolism.
Inborn errors of metabolism are inherited
Determination of the pattern of inheritance of a condition is often helpful in making a diagnosis of genetic disease, and it provides the foundation for genetic counselling. Themostimportantinformationrequiredforestablishingthepatternofinheritance is a family history covering at least three generations of relations.
Autosomal recessive disorders Most of the inherited metabolic diseases recognized today are inherited in the same manner as Garrod’s original inborn errors of metabolism: they are Mendelian, single-gene defects, transmitted in an autosomal recessive manner. Disease expres- sion requires that an individual be homozygous for significant, though not neces- sarily the same, mutations in the same gene. In the overwhelming majority of cases, homozygosity occurs as a result of inheritance of a mutant gene from each parent, who are both heterozygous for the defect. Although it is theoretically possible for one, or even both, of the mutations to arise in the patient as a result of de novo mutation, this is so unlikely that for practical purposes it is ignored. Inheritance of two copies of a mutation from one heterozygous parent may occur as a result of uniparental isodisomy. However, this phenomenon is very rare. Table 1.3 Some examples of inborn errors of metabolism in which secondary metabolic defects play a prominent role in the production of symptoms of disease
Disease Metabolic defect Secondary metabolic abnormalities Main clinical findings
CAH 21-Hydroxylase deficiency Androgen accumulation and deficiencies of Addisonian crisis; virilization of females aldosterone and cortisol GSD type I Glucose-6-phosphatase deficiency Lactic acidosis; hyperuricemia; Massive hepatomegaly; hypoglycemia; hypertriglyceridemia failure to thrive HFI Fructose-1-phosphate aldolase Lactic acidosis; hypoglycemia; hyperuricemia; Severe metabolic acidosis; hypoglycemia deficiency hypophosphatemia Methylmalonic acidemia Methylmalonyl-CoA mutase Hyperammonemia; hyperglycinemia Acuteencephalopathy; metabolic acidosis deficiency HHH syndrome Ornithine transport defect Homocitrullinemia Hyperammonemic encephalopathy OTCdeficiency OTC deficiency Orotic aciduria Hyperammonemic encephalopathy Abetalipoproteinemia Apolipoprotein B deficiency Malabsorption of vitamin E Spinocerebellar degeneration
Abbreviations: CAH, congenital adrenal hyperplasia; GSD, glycogen storage disease; HFI, hereditary fructose intolerance; OTC, ornithine transcarbamoylase; HHH, hyperammonemia-hyperornithinemia-homocitrullinemia. 11 General principles
Most individuals with autosomal recessive inherited metabolic diseases have no family history of the disorder. However, the occurrence of a similar disorder in a sibling or in a cousin raises the possibility that the condition is not only heredi- tary, but that it is transmitted as an autosomal recessive disorder. Obtaining the information may be difficult because the occurrence of serious disease in chil- dren, particularly if it is associated with mental retardation, early infant death, or physical deformities, may be concealed by the family out of shame, or simply forgotten. Consanguinity increases the likelihood that an inherited disorder is autosomal recessive because it increases the probability that both parents of a child are carriers of a rare recessive mutation. As a rule, the more rare it is, the more likely the occurrence of an autosomal recessive condition will be affected by inbreeding. For some very rare disorders, the frequency of consanguinity of the parents of affected individuals is as high as 30–40%. Geographic or socio-cultural isolation of relatively small and demographically stable communities increases the risk of inadvertent inbreeding, no doubt contributing to the high incidence of certain diseases in specific ethnic groups. When considering the possibility that the disease in an individual may be the result of an autosomal recessive mutation, the family history should include specific questions to assess the possibility of parental consanguinity. Simply asking the parents if they are related will often reveal the fact. The origins of the parents are also important. The possibility of consanguinity is increased, for example, if the parents of a patient both come from a small village with a history of population stability and isolation, and if relatives on both sides of the family share the same surname. The increased incidence of a specific genetic defect in a demographically isolated population as a result of the introductionofthe mutation by a founding member is called a ‘founder effect’. The high incidences of certain rare inherited metabolic disorders in specific ethnic groups or communities are well known examples of a putative founder effect, though the role of an element of environmental selection favoring heterozygotes has not been eliminated in some cases. Some examples of inborn errors of metabolism occurring in particularly high frequency in specific ethnic groups are shown in Table 1.4.
X-linked recessive disorders In males, it only takes one mutation of a gene on the X-chromosome to produce disease. Unlike autosomal recessive disorders, in which the contribution of new mutations to the occurrence of disease in individuals is negligible, about a third of males with X-linked recessive diseases are born to mothers who are not carriers of the mutation: the boys are affected as a result of new mutations. For the purposes of genetic counselling, once the medical diagnosis has been confirmed and the 12 A Clinical Guide to Inherited Metabolic Diseases
Table 1.4 Some examples of inborn errors of metabolism occurring in high frequency among specific ethnic groups
Estimated incidence Disease Ethnic group (per 100,000 births)
∗ Tay-Sachs disease Ashkenazi Jews 33 Gaucher disease Ashkenazi Jews 100 ∗ Hepatorenal tyrosinemia French-Canadians (Saguenay-Lac 54 Saint-Jean region) Porphyriavariegata South African (white) 300 Congenital adrenal hyperplasia Yupik Eskimos 200 Phenylketonuria (PKU) Turkish 38.5 Yemenite Jews 19 Ashkenazi Jews 5 Glutaric aciduria, type I Ojibway Indians (Canada) >50† Maple syrup urine disease Mennonites (Pennsylvania) 3.3 Salla disease Finnish 568
Note: ∗Before the introduction of screening and prenatal diagnosis to prevent the condition. †Estimated. Source: Data derived in part from Weatherall, D. J. (1991) and Scriver et al. (1995).
possibilities of autosomal recessive and non-genetic phenocopies have been elimi- nated, it is critical to determine whether the disease caused by an X-linked mutation developed as a result of inheritance of the mutation, or as a result of a new muta- tion. The family history is particularly important in this situation. The likelihood that the mother of a boy with an X-linked recessive disease inherited the mutation from her own mother can be estimated from the number of healthy male relatives she has related to her through her mother and sisters. For example, the mother of a boy with Hunter disease (MPS II), an X-linked recessive mucopolysaccharide storage disease, is unlikely to have inherited the mutation from her own mother if she has a large number of healthy brothers and nephews (i.e., sons of her sisters). The larger the number of healthy male relatives, the more likely the affected boy has the disease as a result of a new mutation, either in the boy himself or in his mother during gametogenesis in each case. In the first situation, the risk of recurrence of the disease in subsequent offspring is negligible. However if the boy’s mother is a carrier as a result of a new mutation, the risk of recurrence is the same as if the mother had inherited the mutation from her mother. It is a mistake to assume automatically that a woman is a carrier of an X-linked disease if she has a son affected with it. But, if a woman has two affected sons, or she has an affected brother as well as an affected son, she is regarded as an obligate 13 General principles
Table 1.5 Some mechanisms of autosomal dominance
Mechanisms Gene product Disease example
Abnormal assembly of the subunits of a Fibrillin Marfan syndrome multimeric protein Abnormal interaction between the Hemoglobin Hemoglobin M disease subunits of multimeric protein Derepression of rate-limiting enzyme Porphobilinogen Acute intermittent porphyria (derepression activity deaminase of -aminolevulinic acid dehydratase) Cell receptor defects LDL-receptor Familial hypercholesterolemia (derepression of HMG-CoA reductase) Cell membrane defects Spectrin Hereditary spherocytosis Deposition of an abnormal structural Transthyretin Hereditary amyloidosis protein Somatic cell mutation coupled with pp110RB Retinoblastoma inheritance of a recessive gene
Abbreviations: LDL, low-density lipoprotein; HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA.
carrier of the disease-causing mutation. It also follows that all the female offspring of a man affected with an X-linked disorder are obligate carriers of the disease; in contrast, none of his sons would be affected because male-to-male transmission of X-linked conditions does not occur.
Autosomal dominant disorders Although autosomal dominant mutations are common causes of genetic disease in humans, they contribute relatively little to the sum total of inherited metabolic disorders. This is probably because, with a few exceptions, most inherited metabolic diseases are caused by abnormalities in enzymes or transport proteins that are not involved in the types of interactions or processes required to produce dominance (Table 1.5). Autosomal dominant inheritance is characterized by: r every affected individual has an affected parent (unless the individual has the disease as a result of a new mutation, or the mutation is non-penetrant); r on average half of the offspring of an affected individual will themselves have only unaffected children (assuming penetrance is complete); r males and females are equally represented among affected members of the kindred; r transmission of the condition occurs vertically through successive generations, unless the condition impairs reproduction. 14 A Clinical Guide to Inherited Metabolic Diseases
Because only one mutation is required to cause disease, new mutations con- tribute significantly to the incidence of autosomal dominant disorders. The rate of spontaneous mutation, and hence the likelihood in any particular situation that disease is due to spontaneous mutation, varies from one disease to another. In some noteworthy cases, the autosomal dominant inheritance of a condition may be over- looked because disease in a particular patient may be the result of a new mutation. Many of the diseases, such as acute intermittent porphyria and dopa-responsive dystonia (Segawa syndrome), are also characterized by incomplete penetrance5 and marked variability in expressivity6,even among members of the same family, further obscuring the dominant mode of inheritance. A careful family history is invaluable in recognizing the familial nature of these disorders.
Mitochondrial inheritance Each mitochondrion in every cell contains several copies of a small, circular, double- stranded DNA molecule (mtDNA) containing genes coding for the production of ribosomal RNA and various tRNAs necessary for mitochondrial protein biosyn- thesis, and for the production of some of the proteins involved in mitochondrial electron transport (Figure 1.2). The mitochondrial genome consists of 16,569 base- pairs, comprising 5523 codons, coding for the production of 37 gene products. The vast majority of mitochondrial proteins, including most of the proteins of subunits involved in electron transport (see Table 9.13), are encoded by nuclear genes. Mutations of these genes cause diseases transmitted as autosomal recessive disorders. As in the case of other autosomal recessive conditions, the disease phe- notype of various affected individuals in the same family tends to be very similar. The situation is quite different with regard to the pattern of inheritance and clinical expression of disease caused by mtDNA mutations. The mitochondria in the cells of each individual are derived at the time of conception from the mitochondria in the cytoplasm of the ovum; the mitochondria and mtDNA of the sperm are lost during the process of fertilization. It follows that mtDNA mutations are also inherited only from the mother. When multiple members of a family are affected with a condition because of inheritance of an mtDNA mutation, the pattern of inheritance is quite specific:
5 Penetrance is the probability that an individual with a specific disease-associated genotype will exhibit clinical manifestations of the disease. It is a statistical conceptgenerally derived from pedigree analysis. Incomplete penetrance refers to the observation that some individuals with a specfic disease-causing mutation may exhibit no clinical manifestations of the disease at all – the disease appears to skip generations in families with multiple affected members. 6 Expressivity is the extent to which individuals with a specific disease-associated genotype, all of whom have disease, exhibit manifestations of the disease. Variable expressivity refers to the observation that several individuals with the same genotype may have disease manifestations varying from very mild to very severe. It is not to be confused with penetrance. 15 General principles
Figure 1.2 The human mitochondrial genome. The human mitochondrial genome is encoded in a double-stranded, circular mtDNA molecule. The figure show in a simplified way the identity and relative locations of vari- ous mitochondrial genes.
r all the offspring of a woman carrying a mtDNA mutation can generally be shown to have inherited the mutation, whether they are clinically affected with disease or not; r the phenotypic expression of disease in different individuals who have inherited mtDNA mutations is often highly variable, both in terms of the systems involved and the severity of clinical disease; r transmission of the condition from father to offspring does not occur. Each cell contains at least hundreds of mitochondria, and any mtDNA may affect all (homoplasmy) or only a fraction (heteroplasmy) of the total mitochondria in each cell. The phenotypic effect of any particular mutation depends on the severity of the mtDNA mutation, the proportion of mitochondria affected, and the suscep- tibility of various tissues to impaired mitochondrial energy metabolism. This makes the relationship between the proportion of mutant mtDNA and clinical phenotype very complex. Owing to different thresholds for susceptibility to mitochondrial 16 A Clinical Guide to Inherited Metabolic Diseases
Figure 1.3 The effect of heteroplasmy on the clinical expression of mtDNA mutations. The figure represents four cells, each containing nine mitochondria. Mitochondria bearing normal mtDNA are shown as open circles; those with a mtDNA mutation are shown as filled circles. As the proportion of mutant mitochondria in the cells of various tissues increases, different thresholds are reached for the production of disease-causing cell damage. While lower proportions of mutant mitochondria are well tolerated, severe disease results when the proportion is very high.
energy defects, the tissues and organs involved in the clinical phenotype may vary markedly from one affected individual to another with the same mtDNA mutation, depending on the degree of heteroplasmy in each individual. Increasingly, fami- lies are being identified in which variations in the extent of the heteroplasmy in the offspring of a clinically healthy woman carrying a specific mtDNA mutation may result in some being clinically completely normal, some, for example, dying in early infancy with severe Leigh disease, and some being affected with clinically intermediate disease variants (mental retardation, retinitis pigmentosa, and ataxia). Figure 1.3 givessome idea of what heteroplasmy is and how it relates to phenotype. Obtaining a family history appropriate to the recognition of this type of inheritance of a specific mtDNA mutation is particularly challenging. No clinical abnormality in a relative, no matter how apparently trivial or how different it may seem from the disease phenotype in the proband, can be dismissed. The mutation rate for mtDNA is much higher than that for nuclear DNA, and the relative contribution of de novo mtDNA mutations especially deletions, to disease is much greater than the contribution of new mutations to disease due to nuclear DNA mutations. Conditions, like Kearns-Sayre syndrome, which is usually the result of mtDNA deletions or duplications, are almost always sporadic, and the risk of recurrence of the condition in the family is low.
Inherited metabolic diseases may present at any age
Historically and traditionally, inheritedmetabolic diseases have been thought of as primarily pediatric problems, even though three of Garrod’s four original inborn 17 General principles
errors of metabolism are generally more commonly identified for the first time in adults rather than in children affected with the disorders. The strong association with pediatrics seems to have grown out of experience with PKU and the possibility that other serious conditions presenting in infancy might be caused by point defects in metabolism. Moreover, as the contribution of infectious diseases and malnutri- tion to pediatric mortality declined over the middle years of the 20th century, the proportion of mortality caused by inbornerrors of metabolism increased rapidly. One thing is certain, reports of the identification and characterization of specific inherited metabolic diseases began to appear in exponentially growing numbers in the pediatric literature long before comparable articles appeared in significant numbers in the general medical literature. Asaresult of advances in diagnostic technology, particularly as it relates to lysosomal and mitochondrial disorders, the number of inherited metabolic dis- eases recognized to present in adulthood has increased enormously. Many, if not most, inborn errors of metabolism originally described in infants and children are now known to occur as adult-onset variants. In most cases, the late-onset vari- ants are clinically milder than variants that are often rapidly fatal in infancy. The clinical presentation and course are often significantly different from those of the pediatric variants (Table 1.6). In some cases, such as milder variants of ornithine transcarbamoylase (OTC) deficiency, the presentation in adults, with severe hyper- ammonemic encephalopathy, often precipitated by severe physiological stress, may resemble that in infants with a similarly lethal outcome. An increasing number of examples are being found of inborn errors of metabolism that seem never to present in childhood. Some are familiar, such as many of the hereditary hyperlipidemias. In others, recognition of the diseases is more recent, such as with Leber hereditary optic neuropathy (LHON) and gyrate atrophy. This is an area that is bound to expand over the coming years.
Three sources of diagnostic confusion
The commonest error in the management of inherited metabolic disorders is prob- ably delayed or wrong diagnosis. There are three common sources of potential confusion in the diagnosis of inherited metabolic disease.
Confusion with common acquired conditions Some inborn errors are often misdiagnosed as acquired disease, particularly some infections, intoxications, or nutritional deficiencies (Table 1.7). Failure to consider both classes of disorders simultaneously in the differential diagnosis of an acutely ill child may result in the loss of an opportunity to carry out critical diagnostic investigations, and may result in unnecessary morbidity, or even death. Table 1.6 Some examples of inborn errors of metabolism presenting in adulthood in which the clinical phenotype differs significantly from that in childhood
Disease Enzyme defect Pediatric phenotype Adult-onset phenotype
GM2 gangliosidosis -hexosaminidase Rapid cognitive (developmental) deterioration, Slowly progressive movement disorder, ataxia, (Tay-Sachs disease with seizures, blindness, progressing to early psychiatric problems, with little or no and Sandhoff disease) death intellectual impairment until late in the course of the disease Gaucher disease Glucocerebrosidase Marked hepatosplenomegaly with early and severe Hepatosplenomegaly, which is often neurological impairment, progressing rapidly asymptomatic, though frequently associated to death (type 2); or hepatosplenomegaly and with hypersplenism, bone pain, fractures, progressive movement disorder, seizures, avascular necrosis of large joints, but no CNS intellectual deterioration, progressing to death involvement (type 1) within a few years (type 3) Niemann-Pick disease Acid sphingomyelinase Marked hepatosplenomegaly with early and severe Hepatosplenomegaly, bone pain, cirrhosis, neurological impairment, progressing rapidly chronic infiltrative lung disease, but no CNS to death (type A) involvement (type B) MPS I -l-iduronidase Dysmorphic facial features, corneal clouding, Little or no facial dysmorphism, corneal dysostosis multiplex, hepatosplenomegaly, clouding, marked restriction of joint valvular heart disease, obstructive pulmonary movement, with early-onset osteoarthritis, disease, marked growth retardation, with severe but no primary CNS involvement (Scheie (Hurler disease), or little or no (Hurler-Scheie disease) disease) CNS involvement MLD Arylsulfatase A Ataxia, developmental delay, cognitive regression, Psychiatric problems primarily affecting progressing rapidly to severe neurological executive function, late slowly progressive impairment and death motor impairment and dementia GSD type 2 Acid maltase (acid Severe, rapidly progressive muscle weakness and Very slowly progressive skeletal muscle weakness -glucosidase) cardiomyopathy with minimal myocardial involvement OTCdeficiency OTC Severe, often lethal, neonatal hyperammonemic Recurrent headaches; rarely presents as severe encephalopathy in affected males; failure to hyperammonemic encephalopathy thrive, recurrent hyperammonemic encephalopathy in affected females LHON mtDNA mutations None Progressive blindness Gyrate atrophy OAT None Progressive blindness NARP mtDNA mutations Leigh disease: failure to thrive, persistent lactic Peripheral neuropathy, retinitis pigmentosa, acidosis, irregularly progressive neurological intellectual impairment, ataxia, with episodes deterioration with episodes of acute of acute deterioration associated with lactic encephalopathy acidosis Adult-onset type II citrin Neonatal hepatitis with intrahepatic cholestasis, Hyperammonemia, disorientation, delirium, citrullinemia generalized amino acidemia (including delusions, stupor progressing to coma, (CTLN2) methionine, tyrosine, citrulline, lysine, associated with marked hypercitrullinemia threonine and arginine), and galactosemia
Abbreviations: CNS, central nervous system; MPS, mucopolysaccharidosis; MLD, metachromatic leukodystrophy; OTC, ornithine transcarbamoylase; LHON, Leber hereditary optic neuropathy; OAT, ornithine aminotransferase; NARP, neuropathy-ataxia-retinitis pigmentosa 20 A Clinical Guide to Inherited Metabolic Diseases
Table 1.7 Some common non-metabolic conditions that are often confused with inherited metabolic diseases
Inherited metabolic ‘syndrome’ Common non-metabolic disease phenocopy
Syndrome (Chapter) Infections Hepatic syndrome (4) Hepatitis, enterovirus infection, infectious mononucleosis Cardiomyopathy (5) Enterovirus infection Storage syndrome (6) Congenital CMV infection, congenital toxoplasmosis Encephalopathy (2) Arbovirus infections, enterovirus infections, herpes infections (especially newborn), postinfectious encephalopathy (e.g., chicken pox)
Intoxications Neurologic syndrome (2) CNS depressants, antihistaminics, anticonvulsants Lactic acidosis (3) Ethanol, methanol, ethylene glycol, salicylism Hepatic syndrome (4) Valproic acid intoxication, amiodarone reaction Cardiac syndrome (5) ACTH reaction (cardiomyopathy)
Nutritional deficiencies Lactic acidosis (3) Thiamine deficiency
Methylmalonic acidemia (3) Vitamin B12 deficiency
Hematopoietic disorders Storage syndrome (6) FEL, hemoglobinopathies, lymphoma, malignant histiocytosis Hepatic syndrome (4)
Abbreviations: CMV, cytomegalovirus; CNS, central nervous system; FEL, familial erythrophago- cytic lymphohistiocytosis.
Confusion caused by association with intercurrent illness Metabolic decompensation in a child withamarginally compensated inherited metabolic disorder commonly occurs as a result of the physiological stress of inter- current illness. Preoccupation with the intercurrent illness often delays diagnosis of the underlying genetic disorder. Owing to an impaired ability to compensate adequately for the metabolic pressures caused by intercurrent illness, particularly infection, children with inherited metabolic diseases often decompensate when they contract relatively trivial infections. The child with intermittent MSUD (maple syrup urine disease) or a fatty acid oxidation defect, or the girl with OTC (ornithine transcarbamylase) deficiency, is often the one in the family who is described as ‘sickly’. They get sicker and take longer to recover from trivial viral infections than their healthy siblings. 21 General principles
However, some inherited metabolic diseases significantly increase the risk of intercurrent illness. For example, recurrent, treatment-resistant, otitis media is a common problem in children of all ages with mucopolysaccharide storage diseases in which distortion of the Eustachian tubes and the production of particularly tena- cious mucus combine to create a favorable environment for bacterial colonization of the middle ear. The neutropenia that is a prominent feature of glycogen disease (GSD), type Ib, and some of the organic acidopathies, predisposes to pyogenic infections. Classical galactosemia predisposes infants to neonatal Escherichia coli sepsis by a mechanism that is not yet understood.
Confusion arising from genetic heterogeneity Among the inherited metabolic diseases, two or more clinically similar disorders may be caused by mutations in completely different genes. This follows from the fact that the net result of a defect in any one of a number of steps in a complex metabolic process may be functionally the same. A prominent example of this is the mucopolysaccharide storage disease, Sanfilippo disease, a group of clinical indistinguishable diseases caused by defects in different enzymes involved in the breakdown of the glycosaminoglycan, heparan sulfate. This has important impli- cations for carrier testing and for prenatal diagnosis, situations in which major decisions are made on the strength of the results of a single laboratory test. Doing the wrong test has a high probability of producing the wrong results, sometimes with tragic consequences.
Congenital malformations and inborn errors of metabolism
On the one hand, major congenital malformations, such as meningomyelocele, complex congenital heart disease, and major congenital limb deformities, are not generally considered signs of an underlying inherited metabolic disease. On the other hand, the recent discovery of a specific defect in cholesterol biosynthesis in patients with Smith-Lemli-Opitz syndrome has forced some modification of this view. There are some inherited metabolic conditions in which dysmorphism is so characteristic that a strong presumptive diagnosis can be made on physical examination alone. This is discussed in detail in Chapter 6.
The internet is particularly important
The internet has revolutionized the management of inherited metabolic diseases by making vast amounts of information readily available – not only to physicians, but to patients, as well. The number of newly recognized disorders, new developments Table 1.8 A list of a selection of Internet websites related to inherited metabolic disorders, either for health professionals or patients and their families
Website Organization maintaining the site Comments Websites with extensive resource directories www.ncbi.nlm.nih.gov/entrez/query.fcgi National Library of Medicine Excellent search engine for articles and abstracts in vast array of journals dating back over 50 years www.genetests.org University of Washington Resource for information on clinics and laboratories providing genetic diagnostic services www3.ncbi.nlm.nih.gov/Omim National Center for Biotechnology Information The ‘gold standard’ for information on the genetics of hereditary single gene disorders www.slh.wisc.edu/newborn/guide Wisconsin State Laboratory ofHygiene Health professionals’ guide to newborn screening www.ninds.nih.gov/health and medical/ National Institute of Neurological Disorders and Information and links related to several inherited metabolic diseases disorders Stroke www.umm.edu/glossary/ University of Maryland Medicine Information and links related to several inherited metabolic diseases archive.uwcm.ac.uk/uwcm/mg/docs/ Cardiff Human Gene Mutation Database Index of locus-specific mutation databases with many links oth mut.html www.ulf.org United Leukodystrophy Foundation Information and links related to several inherited metabolic neurodegenerative diseases www.rarediseases.org National Organization for Rare Disorders (NORD) Information and links related to several inherited diseases www.ntsad.org National Tay-Sachs and Allied Diseases Association Information and links related to several inherited metabolic neurodegenerative diseases www.nfjgd.org National Foundation of Jewish Genetic Diseases Information and links related to several inherited metabolic diseases prevalent among Ashkenazi Jews mcrcr2.med.nyu.edu/murphp01/ NewYork University Medical Center International directory to many genetic diseases and support groups frame.htm Disease-specific websites www.oaanews.org Organic Acidemia Association, Inc. Information on several inherited disorders of organic acid metabolism www.pkunews.org National PKU News Information on PKU for patients, parents and professionals www.nucdf.org National Urea Cycle Disorders Foundation Information on urea cycle disorders for patients, parents and professionals www.msud-support.org Maple Syrup Urine Disease Family Support Group Information on MSUD for patients, parents and professionals www.galactosemia.org Parents of Galactosemic Children, Inc. Information on galactosemia primarily for patients and parents www.agsd.uk/home/ Association for Glycogen Storage Disease (UK) Information on glycogen storage diseases for patients, parents and professionals www.lsdn.com Lysosomal Storage Disease Network Information and links related to many lysosomal storage diseases www.lda.org.au Lysosomal Diseases Australia Information and links related to many lysosomal storage diseases www.ldnz.org.nz New Zealand LSD Support Group Information and links related to many lysosomal storage diseases www.umdf.org United Mitochondrial Disease Foundation Information and links related to various mitochondrial diseases www.wilsonsdisease.org Wilson’s Disease Association International Information and links related to many Wilson diseases www.mpssociety.org National MPS Society, Inc. Informationonmucopolysaccharide storage diseases for patients, parents and professionals www.nnpdf.org National Niemann-Pick Disease Foundation, Inc. Information on Niemann-Pick disease for patients, parents and professionals www.meadjohnson.com/metabolics/ Mead Johnson Nutritionals Information and links related to several inherited metabolic diseases metabolichandbook.html treatable by dietary manipulation www.bdsra.org Batten Disease Support and Research Association Information and links related to neuronal ceroid lipofuscinosis (Batten disease) www.fabry.org Fabry Support and Information Group Information and links related to Fabry disease www.gaucherdisease.org National Gaucher Foundation Information and links related to Gaucher disease www.canavanfoundation.org Canavan Foundation Information and links related to Canavan disease www.canavan.org Canavan Research Foundation www.fodsupport.org Fatty Oxidation Disorder Communication Network Information and links related to various fatty acid oxidation disorders www.worldpompe.org/index.html International Pompe Association A federation of patients’ groups with information and links related to Pompe disease Table 1.9 A selection of locus-specific mutation databases
Disease Gene Website Institution maintaining the site
X-linked ABCD1 www.x-ald.nl Academic Medical Center, Amsterdam, Netherlands adrenoleukodystrophy and www.peroxisome.org Kennedy Krieger Institute, Baltimore, MD Hereditary fructose ALDH9 www.bu.edu/aldolase Boston University, Boston, MA intolerance Congenital disorders of ALG6, DPM1, GCS1, MGAT2, www.kuleuven.ac.be/med/cdg Leuven University, Leuven, Belgium glycosylation not561, PMM2 Hypophosphatasia ALPL www.sesep.uvsq.fr/Database.html University of Versailles-Saint Quentin en Yvelines, France Wilson disease ATP7B www.medgen.med.ualberta.ca/database.html University of Alberta, Edmonton, Canada and life2.tau.ac.il/GeneDis/Tables/Wilson/Wilson.html Tel-Aviv University, Tel-Aviv, Israel Homocystinuria CBS www.uchsc.edu/sm/cbs/cbsdata/cbsmain.html University of Colorado Health Sciences Center, Denver, CO Albinism CHS1, TYR, TYRP1 www.cbc.umn.edu/tad/tyasemut.html University of Minnesota, Minneapolis, MN www.cbc.umn.edu/tad/tyr1mut.html www.retina-international.com/sci-news/tyrmut.htm Retina International Chediak-Higashi CHS1 www.cbc.umn.edu/tad/chsmut.html University of Minnesota, Minneapolis, MN syndrome and www.retina-international.com/Sci-news/chsmut.htm Retina International Neuronal ceroid CLN2, CLN3, CLN5, CLN8, www.ucl.ac.uk/ncl University College London, UK lipofuscinosis (Batten PPT1 (CLN1) and disease) www.retina-international.com/Sci-news/cln3mut.htm Retina International G6PD deficiency G6PD rialto.com/favism/mutat.htm Scripps Research Institute, La Jolla, CA Pompe disease (GSD II) GAA www.eur.nl/FGG/CH1/pompe Erasmus University, Rotterdam, Netherlands Krabbe globoid cell GALC life2.tau.ac.il/GeneDis/Tables/Krabbe/krabbe.html Tel-Aviv University, Tel-Aviv, Israel leukodystrophy Galactosemia GALT www.ich.bris.ac.uk/galtdb Institute of Child Health, Bristol, UK and www.emory.edu/PEDIATRICS/medgen/ Emory University, Altanta, GA research/galt.htm Gaucher disease GBA www.tau.ac.il/∼racheli/genedis/gaucher/gaucher.html Tel-Aviv University, Tel-Aviv, Israel Phenylketonuria (PKU) PAH ww2.mcgill.ca/pahdb McGill University, Montreal, Canada Dihydropteridine QDPR www.bh4.org University Children’s Hospital, Zurich, Switzerland reductase deficiency Pterin-4a-carbinolamine PCBD www.bh4.org University Children’s Hospital, Zurich, Switzerland dehydratase deficiency GTP cyclohydrolase I GCH1 www.bh4.org University Children’s Hospital, Zurich, Switzerland deficiency 6-Pyruvoyloo- PTS www.bh4.org University Children’s Hospital, Zurich, Switzerland tetrahydropterin synthase deficiency GM2 gangliosidosis GMA data.mch.mcgill.ca/gm2adb McGill University, Montreal, Canada Tay-Sachs disease HEXA data.mch.mcgill.ca/hexadb McGill University, Montreal, Canada and life2.tau.ac.il/GeneDis/Tables/Tay Sachs/ Tel-Aviv University, Tel-Aviv, Israel tay sachs.html Sandhoff disease HEXB data.mch.mcgill.ca/hexbdb McGill University, Montreal, Canada Lesch-Nyhan disease HPRT1 www.ibilio.org/dnam/mainpage.html University of North Carolina, NC Congenital adrenal HSD3B2 life2.tau.ac.il/GeneDis/Tables/CAH/cah.html Tel-Aviv University, Tel-Aviv, Israel hyperplasia Familial LDLR www.ucl.ac.uk/fh University College London, UK hypercholesterolemia and www.umd.necker.fr Hopitalˆ Necker-Enfantrs Malades, Paris, France Sanfilippo disease, type B NAGLU www.peds.umn.edu/gene/mutation University of Minnesota, Minneapolis, MN Lowe syndrome OCRL www.nhgri.nih.gov/DIR/GDRB/Lowe The National Human Genome Research Institute, Bethesda, MD Ornithine OTC 63.75.201.100/otc University of Minnesota, Minneapolis, MN transcarbamoylase deficiency X-linked hypophosphatemia PHEX data.mch.mcgill.ca/phexdb McGill University, Montreal, Canada Cystinuria SLC3A1 data.mch.mcgill.ca/cysdb McGill University, Montreal, Canada Mitochondrial cytopathies Various www.gen.emory.edu/mitomap.html Emory University, Atlanta, GA 26 A Clinical Guide to Inherited Metabolic Diseases
in diagnostic technology, and advances in treatment are growing at such a rapid rate that staying abreast of the field is a daunting task. Information available through the internet includes: r peer-reviewed journal articles. Various search engines are available which allow the reader to find in a matter of moments specific articles on a subject, by searching on keyword, author, or journal. A particularly use- ful site is that maintained by the National Library of Medicine of the U.S. (www.ncbi.nlm.nih.gov/entrez/query.fcgi). Avast array of journals is accessible by this technique, covering over 50 years of publications. Access to abstracts is generally free. Some journals also provide full text versions of the articles free of charge, though most require the reader to have an individual or institutional subscription, or to pay for access. This is a particularly valuable resource for physicians. r Disease-specificfoundationsites.Thesesitescangenerallybefoundbyconducting ageneral search on the name of the disease or class of diseases of interest. A partial list is provided in Table 1.8.The material provided by these sites generally includes awide range of information, from very basic descriptions and explanations in layman terms to references to the medical-scientific literature for physicians. Most also provide the opportunity for readers, including patients, to ask questions about the disease of interest. Some provide ‘bulletin boards’ for the exchange of information, opinions, and experiences between readers. These are particularly valuable sites, for patients, as well as for treating physicians. r Medical-scientific databases. A number of sites has been developed to gather and collate technical information of relevance to specific technical aspects of inherited metabolic diseases. These include databases reporting the current status of the identification and characterization ofmutations associated with specific diseases. Mutation analysis has become particularly important for carrier detection and prenatal diagnosis of inherited metabolic diseases for which specific mutations have been described. A partial list of these sites is provided in Table 1.9. r Diagnostic services sites. GeneTests (www.genetests.org/) is a website funded by the National Institutes of Health of the U.S. providing information on a range of services provided by clinics and laboratories for the diagnosis of genetic diseases. Of specific interest is the international laboratory directory of genetic testing laboratories offering accredited laboratory testing for specific diseases, including mutation analyses. r Metabolic list-serve. This is an extraordinarily useful electronic forum pro- viding physicians around the world with the opportunity to exchange ques- tions and viewpoints, including unpublished experience, related to inherited metabolic diseases. It is accessed by application to the administrator at metab-l- [email protected]. 27 General principles
SUGGESTED READING
Blau, N., Duran, M., Blaskovics, M. E. & Gibson, K. M. (eds). (2003). Physician’s Guide to the Laboratory Diagnosis of Metabolic Diseases,2nd ed, Heidelberg: Springer-Verlag. DiMauro, S. & Schon, E. A. (2003). Mitochondrial respiratory-chain diseases. NewEngland Journal of Medicine, 348, 2656–68. Fernandes, J. Saudubray, J. M. & Van den Berghe, G. (eds). (2000). Inborn Metabolic Diseases,3rd ed, Heidelberg: Springer-Verlag. Gilbert-Barness, E. & Barness, L. A. (2000). Metabolic Diseases: Foundations of Clinical Manage- ment, Genetics, and Pathology.Natick, MA: Eaton Publishing Co. Guttmacher, A. E., Collins, F. S. & Carmona, R. H. (2004). The family history – more important than ever. NewEngland Journal of Medicine, 351, 2333–6. Hoffmann, G. F., Nyhan, W. L., Zschocke, J., Kahler, S. G. & Mayatepek, E. (2001) Inherited Metabolic Diseases, Philadelphia: Lippincott Williams & Wilkins. Korf, B. R. (2000). Human Genetics. A Problem-Based Approach,2nd ed, Malden, Massachusetts: Blackwell Science Inc. Nyhan, W.L. & Ozand, P.T.(1998). Atlas of Metabolic Diseases.London: Chapman & Hall Medical. Saudubray, J. M., Ogier, H. & Charpentier, C. Clinical approach to inherited metabolic diseases. In:Fernandes, J., Saudubray, J. M. & Van den Berghe, G. (eds). (1996). Inborn Metabolic Diseases: Diagnosis and Treatment,2nd ed. Berlin: Springer-Verlag, pp 3–39. Scriver, C. R., Beaudet, A. L., Sly, W. S. & Valle D. (eds). (2001). The Metabolic and Molecular Bases ofInherited Disease,8th ed, New York: McGraw-Hill. Thorburn, D. R. (2004). Mitochondrial disorders: prevalence, myths and advances. Journal of Inherited Metabolic Diseases, 27, 349–62. Vogel, F. & Motulsky, A. G. (1996). Human Genetics,3rd ed, Berlin: Springer-Verlag. Zschocke, J. & Hoffmann, G. F.(2004). Vademecum metabolicum: manual of metabolic paediatrics, 2nd ed, Stuttgart: Schattauer. 2 2 Neurologic syndrome
Neurologic symptoms are the presenting and most prominent clinical problems associated with many inherited metabolic disorders. However, neurologic problems in general are common, especially psychomotor retardation, and deciding whom to investigate, and the type of testing to be done, is often difficult. The age of onset and clinical course often provide important clues to the metabolic nature of the disorder. This is also one situation in which delineation of the extent of the pathology is often invaluable. Besides determining the range of pathology within the nervous system, it is important to establish the extent to which other organs and tissues are involved in order to make a rapid diagnosis of inherited metabolic disease. Careful and comprehensive clinical assessment, along with imaging studies, elec- trophysiologic investigation, and histopathologic and ultrastructural information from selected biopsies help to establish the distribution and type of abnormalities within the nervous system. Some patterns of abnormalities are so typical of certain disorders that metabolic studies are required only to confirm the diagnosis. Simi- larly, the pattern and degree of involvement of other organs and tissues is sometimes sufficiently characteristic to suggest a specific course of metabolic investigation. On one hand, for example, the presence of retinitis pigmentosa, hepatocellular dys- function, and renal tubular dysfunction, in a child with psychomotor retardation, muscle weakness and seizures, strongly suggest the possibility of a mitochondrial defect. On the other hand, the presence of hepatosplenomegaly without significant hepatocellular dysfunction in a child with slowly progressive psychomotor retar- dation and ataxia without seizures suggests that the pursuit of a diagnosis of a lysosomal storage disease is likely to be more productive. Among the inherited metabolic diseases, there are six particularly common neu- rologic presentations: r Chronic encephalopathy r Acute encephalopathy r Stroke 28 29 Neurologic syndrome
r Movement disorder r Myopathy r Psychiatric or behavioral abnormalities
Chronic encephalopathy – without non-neural involvement
Whether the signs of disease are primarily signs of gray matter or white matter involvement, or both, is a useful guide to diagnostic investigation.
Gray matter disease (poliodystrophy) Psychomotor retardation or dementia, seizures, impairment of special senses, such as blindness, and extrapyramidal disturbances generally occur early in the course of gray matter diseases.
Psychomotor retardation or dementia Of all the neurologic problems that occur in patients with inherited metabolic diseases, developmental delay or psychomotor retardation is the commonest. The diagnosis of psychomotor retardation involves assessment of age appropriateness in anumber of developmental spheres, including IQ in older patients, and gross motor, fine motor, socio-adaptive, and linguistic milestones in young children and infants. In young children, the Denver Developmental Screening Test is relatively easy to master and apply on a routine basis. Other screening tests are more sophisticated and require special training or access to special supplies or equipment. The periodic reports provided by teachers on the social and academic progress of a child in class provide invaluable information on development, particularly on any deterioration over a period of several months. In adults, chronic encephalopathy may take the form of progressive dementia. Early recognition and assessment of the progression of the disorder is facilitated by use of any one of a number of mini-mental state examinations, such as the Folstein Mini Mental State Examination (MMSE). They are short, easy to execute and score, and require no special equipment to administer. They are useful as screening instruments; however, in order for any clinical test of higher integrative functioning to be reliably interpretable, the afferent and efferent components of any task must be intact. In other words, the application of tests requiring the patient to be able to see are worthless in patients who are blind. Similarly, the performance of complex actions requiring coordination, such as drawing shapes, are uninterpretable in patients with severe motor disturbances, whether higher integrative functions are intact or not. Screening tests of mental state are unreliable in individuals who are not fluently comfortable speaking English, or in individuals with little formal education. They are also relatively insensitive to defects in executive functioning, which often emerge 30 A Clinical Guide to Inherited Metabolic Diseases
before other signs of dementia, but require the use of more sophisticated testing, such as Raven’s Progressive Matrices, to identify. The clock-drawing test is more sensitive than the MMSE as a screening test for executive dysfunction, and it is easy to administer in the clinic or at the bedside. However, it does take some practice to learn how to score it. Psychomotor retardation is a prominent feature of many inherited metabolic diseases presenting in childhood, but only a fraction of the mental retardation encountered in practice will turn out to be caused by inborn errors of metabolism. Who, then, should be investigated, and what type of investigation is most appro- priate in each case?
Some general characteristics of the psychomotor retardation caused by inborn errors of metabolism in children There are some characteristics of the cognitive disabilities caused by inherited metabolic disease which, when present, should alert the clinician to the possibility of an underlying inborn error of metabolism. First, it tends to be global, affecting all spheres of development to some extent. Although a mild developmental problem may present as speech delay, in most cases, a careful history and developmental examination show that the defect extends to other developmental spheres. Older children with mental retardation caused by inborn errors of metabolism commonly show discrepancies in performance on tests of general intelligence, such as the Wechsler Intelligence Scale for Children (revised) (WISC-R): they often perform better on tests of verbal skills compared with motor skills. On the other hand, conditions characterized by progressive myopathy may present as developmental delay characterized by deficits limited to gross motor activities. The nature of the underlying disability usually becomes obvious on physical examination. Secondly, severe irritability, impulsivity, aggressiveness, and hyperactivity are also more common among infants with mental retardation caused by inborn errors of metabolism than among infants with nonmetabolic diseases. Infants with Krabbe globoid cell leukodystrophy are often implacable. Patients with Sanfilippo disease (MPS III) and boys with Hunter disease (MPS II) exhibit particularly disruptive behavior, which in the case of Sanfilippo disease may be the presenting complaint. Motorautomatisms and stereotypic behavior are also common in these disor- ders. Compulsive chewing of the thumb and fingers often results in maceration of the skin and chronic paronychia. The self-mutilatory behavior of boys with Lesch- Nyhan syndrome (X-linked HPRT deficiency) is particularly prominent, sometimes resulting in traumatic amputation of fingers or severe laceration of the lips. Noc- turnal restlessness is a common problem in both children and adults with inherited metabolic diseases affecting the brain. 31 Neurologic syndrome
Thirdly, the psychomotor retardation is usually progressive.Thereis generally a history of a period of apparently normal development, followed by loss of develop- mental milestones or progressive deterioration in school performance. Initially, the progression may be subtle, amounting to an apparent arrest of development during which the gap between the developmental level of the patient and normal children of the same age grows wider with time, without any obvious loss of developmen- tal milestones. Ultimately, loss of previously acquired skills makes the progressive nature of the problem obvious. On the one hand, earlier onset signals a more rapidly progressive course of the mental handicap. The developmental deficit in a six-year-old with a history of mild mental retardation dating from early infancy, and associated with no regression or other neurologic problems, is unlikely to be attributable to any known inherited metabolic disease. On the other hand, the progression of the intellectual deficit in late-onset GM2 gangliosidosis is usually very slow, tending to be obscured by the prominence of the movement disorder or psychiatric problems associated with the disease. The course of the deterioration in some inherited metabolic diseases, such as metachromatic leukodystrophy, is sigmoidal: a period of relatively slow progression is followed by rapid deterioration, which is then followed by a long period in a relatively stable near-vegetative state. It is important to distinguish primary developmental regression, occurring as a result of progression of the disease, from pseudo-regression due to environmental or other secondary effects on the nervous system (Table 2.1). Fourthly, the psychomotor retardation is usually associated with other objective evidence of neurologic dysfunction,such as disorders of tone, impairment of special senses,seizures,pyramidaltractsigns,evidenceofextrapyramidaldeficits,orcranial nerve deficits. Moreover, the likelihood that the mental retardation is due to an inborn error of metabolism is increased if the associated neurologic deficits involve more than one part of the nervous system, such as evidence of central nervous system (CNS) disease, along with signs of a peripheral neuropathy. Ageneral approach to the investigation of inherited metabolic causes of chronic encephalopathy is presented in Figure 2.1.Itisbased on the early determination of the extent of involvement of non-neural tissues and the degree of involvement of different components of the nervous system. Those disorders in which metabolic acidosis is a prominent aspect of the presentation are discussed in Chapter 3.Simi- larly, conditions in which hepatic involvement dominates the clinical presentation are considered in Chapter 4, and conditions typically associated with unusual phys- ical features of dysmorphism are discussed in Chapter 6. This approach serves well when the clinical manifestations of disease are well established, but many of the signs that are particularly characteristic of inherited metabolic diseases only emerge with observation over a period of time. 32 A Clinical Guide to Inherited Metabolic Diseases
Table 2.1 Causes of developmental pseudo-regression
Emotional problems, such as depression The apparent developmental regression of emotionally disturbed infants is well-recognized. This is not a common cause of pseudo-regression in very young children, but must be considered in patients who are mature and lucid enough to be aware of their advancing disease.
Poorly controlled seizure activity Apparent developmental regression is a common consequence of poor seizure control. The problem is particularly difficult to unravel when the seizures themselves are clinically subtle, but frequent enough to impair consciousness for significant periods of time.
Over-medication with anticonvulsants The relationship between apparent regression and the introduction of new drugs or changes in drug dosages is usually obvious. An understanding of the usual course of the response to anticonvulsant therapy and possible drug interactions (e.g., erythromycin and carbamazepine), helps to identify this common cause of pseudo-regression.
Intercurrent systemic illness Children with severe static brain lesions, such as cerebral palsy, often show developmental regression during intercurrent systemic illnesses. This is generally recognized to be reversible in time. However, the recovery of skills is sometimes so slow it raises the question of possible neurological regression that may prompt needless investigation. The relationship to intercurrent illness is usually obvious.
Secondary neurological problems Secondary neurological problems arising as part of the natural history of some static brain lesions may result in the loss of some previously acquired skills. One example is the loss of mobility arising from skeletal and joint deformities caused by spasticity. A previously ambulatory child with cerebral palsy may stop walking as a result of shortening of the Achilles tendons. The resulting discrepancy between gross motor and other developmental spheres is a clue to the mechanism of the regression in these patients.
Initial investigation Astrategy for the initial investigation of young patients presenting with what might be regarded as undifferentiated chronic encephalopathy or psychomotor retarda- tion without evidence of non-neurologic involvement is shown in Table 2.2.It includes studies: r to determine the extent and degree of neurologic damage, r to ensure that the early stages of some treatable metabolic disorder are not missed, and r studies to establish a baseline for monitoring the natural history of the condition. Anymetabolic abnormality would be an indication for further investigation. Pri- mary disorders of amino acid metabolism would be unlikely to be missed through this approach. Similarly, most primary defects of organic acid metabolism would 33 Neurologic syndrome
Figure 2.1 An approach to inherited metabolic diseases with chronic encephalopathy. Abbreviations: NCL, neuronal ceroid-lipofuscinosis; CRSM, cherry-red spot-myoclonus syn- drome; MELAS, mitochondrial encephalomypathy-lactic acidosis and stroke-like episodes syndrome; XLALD, X-linked adrenoleukodystrophy; AAurias, amino acidurias; OAurias, organic acidurias; MLD, metachromatic leukodystrophy; GLD, globoid cell leukodystrophy; NPD, Niemann-Pick disease; MPS, mucopolysaccharidosis; MSD, multiple sulfatase defi- ciency; HSM, hepatosplenomegaly.
be detected, particularly those in which the psychomotor retardation is severe. It is impossible to exaggerate the importance of imaging techniques, especially MRI with spectroscopy, in the diagnosis of inherited metabolic diseases presenting as neurological syndrome. Many of these conditions are associated with imaging abnormalities that are so characteristic they are virtually diagnostic. Marked reduc- tion in the size of the creatine peak on magnetic resonance spectroscopy of the brain may be the only significant laboratory finding in patients with defects of creatine biosynthesis or in boys with X-linked creatine transporter defects causing severe developmental delay and hypotonia. The schedule and protocol for reassessment would depend primarily on the age of the patient, the severity of the psychomotor disability, the findings at the initial investigation, and the reproductive plans of the parents. One must be alert to the 34 A Clinical Guide to Inherited Metabolic Diseases
Table 2.2 Initial investigation of chronic encephalopathy
Thorough developmental assessment and neurologic examination Brain imaging: CT or MRI scan Electrophysiologic studies: auditory brain stem responses, visual evoked potentials, somatosensory evoked potentials, nerve conduction studies, EMG Radiographs of the hands, chest, and lateral of the spine: for evidence of dysostosis multiplex (see Chapter 6) Plasma amino acid analysis: screening by thin-layer chromatography will meet most needs; quantitative amino acid analysis, if abnormalities are found (see Chapter 9) Urinary amino acid thin-layer or paper chromatography Urinary organic acid analysis, even in the absence of overt metabolic acidosis Plasma ammonium, preferably two hours after a normal meal of protein-containing food Plasma lactate Urinary MPS screening test (see Chapter 9) Urinary oligosaccharide screening test (see Chapter 9)
Abbreviations: MPS, mucopolysaccharide; EMG, electromyography.
emergence of new clinical signs and be prepared to depart from a protocol that might have been generated in the first place by the feeling that the problem was not the result of an inborn error of metabolism. Often the clinical signs of disease at presentation in early childhood may suggest an inherited metabolic disorder, but intensive metabolic investigation fails to demonstrate any diagnostically specific abnormality, and the subsequent clinical course of the disease turns out to be more consistent with a static neurological lesion. In other cases, like Rett syndrome, the subsequent clinical course of the condition is sufficiently typical to indicate the diagnosis. In other situations, new information may emerge that redirects the metabolic investigation, leading to the identification of a specific primary metabolic abnormality or a new disease.
Seizures Recurrent seizures by themselves, in the absence of other evidence of brain disease or systemic metabolic abnormalities, such as hypoglycemia, are unusual as the first manifestation of inherited metabolic diseases. The specific characteristics of seizures that might suggest they are in fact the result of a primary disorder of brain metabolism include: r onset early in life. r association with other neurologic signs, such as psychomotor retardation, disor- ders of tone, movement disorders, or visual impairment. r complex partial or myoclonic seizures. r resistance to conventional anticonvulsant therapy. 35 Neurologic syndrome
Intractable seizures in the newborn are considered in detail in Chapter 7.Beyond the newborn period, there is a small number of inherited metabolic diseases in which presentation primarily as a seizure disorder is common (Table 2.3), perhaps with little or no evidence of other problems. In these, the seizure phenotype often resembles West syndrome, with a mixture of partial complex seizures, absence attacks, and frequent massive myoclonic jerks. One of the most difficult of this category is the group of patients with atypical pyridoxine-dependent seizures. Pyridoxine-dependent seizures typically present in the newborn period (Chapter 7)asgeneralized tonic-clonic seizures, which are dramatically responsive to administration of large intravenous doses (100 mg)
of pyridoxine (vitamin B6). The diagnosis of atypical pyridoxine dependency is also based on the response to treatment with pyridoxine; however, the response is slower and more variable. Rapid response to therapy seems to be the exception,
and exclusion of the diagnosis may require a trial of up to 50 mg of vitamin B6 per kilogram of body weight, given daily for at least three weeks. Biotinidase deficiency, a form of multiple carboxylase deficiency, commonly presents between three and six months of life with failure to thrive, metabolic acidosis, a skin rash resembling seborrheic dermatitis, and alopecia, in addition to seizures (see Chapter 3). However, any of the usual features of the disorder may be absent. Some infants have been reported presenting as early as one month of age with infantile spasms. The skin rash, hair changes, and acidosis may only develop some weeks or months later. Presumptive diagnosis is by urinary organic acid analysis, though in some infants the typical abnormalities are sometimes absent. Confirmation of the diagnosis is by enzyme assay on as little as a few drops of blood. The response to treatment with biotin is dramatic. If the diagnosis is considered, treatment with 20 mg per day should be begun without delay while awaiting the results of laboratory studies. Seizures are generally the only early sign of inherited defects in glucose transport across the blood-brain barrier, caused by mutations in the GLUT1 gene. Infants with this disorder characteristically present a few months after birth with a history of complex partial, myoclonic, or absence seizures that are typically resistant to conventional anticonvulsant medication. Routine biochemical studies of blood and urine are normal. The EEG and imaging studies are also often normal. However, simultaneous measurement of plasma and CSF shows hypoglycorrhachia: the ratio of CSF to plasma glucose, which is normally >0.65, is decreased to < 0.35. This condition generally responds well to treatment with a ketogenic diet. Seizures may be the presenting sign of early-onset variants of neuronal ceroid- lipofuscinosis (NCL). They are invariably a major problem in the later stages of the disease, regardless of the age of onset. Developmental delay, psychomotor regression, or dementia are almost always present, usually preceding the onset of myoclonus, which may be interpreted as seizures. Visual impairment progressing Table 2.3 Inherited metabolic disease in which seizures are particularly prominent in the absence of obvious non-neural involvement
Disease Other clinical features Diagnosis
Pyridoxine dependency Neonatal or early infantile onset of intractable tonic-clonic Absence of metabolic acidosis or specific abnormalities of seizures intermediary metabolism; rapid EEG and clinical response to pyridoxine. Atypical pyridoxine dependency Early infantile onset of intractable tonic-clonic seizures Absence of metabolic acidosis or specific abnormalitiesof intermediary metabolism; delayed clinical response to pyridoxine without significant EEG changes. Tay-Sachs disease Developmental arrest, hypotonia, visual inattentiveness, Deficiency of -hexosaminidase A in serum, leukocytes, or markedly exaggerated startle reflex, and macrocephaly fibroblasts. Sandhoff disease Developmental arrest, hypotonia, visual inattentiveness, Deficiency of total -hexosaminidase in serum, leukocytes, markedly exaggerated startle reflex, and macrocephaly or fibroblasts. Multiple carboxylase deficiency Early infantile onset of intractable tonic-clonic seizures, often Urinary organic acids generally, though not always, show with skin rash, alopecia, and lactic acidosis presence of 3-methylcrotonate, 3-methylcrotonylglycine, 3-hydroxyisovalerate, and other metabolites (see Chapter 3) Infantile NCL (Santavuori-Hagberg Early infantile-onset psychomotor retardation, myoclonic Typical lysosomal inclusions seen on electron microscopic syndrome) seizures, blindness, early flattening of the EEG examination of skin, leukocytes, or conjunctival epithelium; mutation analysis Late-infantile NCL Onset of partial complex and myoclonic seizures at 2–4 years Typical lysosomal inclusions seen on electron microscopic (Jansky-Bielschowsky syndrome) of age, progressive visual impairment, developmental examination of skin, leukocytes, or conjunctival regression, ataxia and tremor epithelium; mutation analysis Juvenile NCL (Spielmeyer-Vogt Tonic-clonic seizures, usually preceded by visual failure, and Typical lysosomal inclusions seen on electron microscopic syndrome) later onset of intellectual regression examination of skin, leukocytes, or conjunctival epithelium; mutation analysis MELAS Lactic acidosis, small stature, seizures, stroke, cortical Defects in mitochondrial ETC in fibroblasts or skeletal blindness, psychomotor retardation muscle; mitochondrial mutation analysis (see Chapter 9) Late-onset galactosialidosis Myoclonus, seizures, corneal clouding, cherry-red spots, Deficiency of -galactosidase and -neuraminidase in mental retardation fibroblasts Krabbe GLD Marked irritability, generalized hypertonia, feeding Deficiency of galactocerebrosidase in leukocytes or difficulties, partial complex seizures fibroblasts 3-Phosphoglyceerate Microcephaly, early-onset severe psychomotor retardation, Abnormally low concentrations of glycine and serine in dehydrogenase deficiency mixed-type seizures, i.e., tonic, clonic, absence, myoclonic) plasma and CSF; low CSF 5-methylTHF; deficiency of enzyme in cultured fibroblasts Peroxisomal disorders Early-onset, marked failure to thrive, severe developmental Elevated VLCFA in plasma delay, partial complex seizures GAMT deficiency Early-onset developmental delay, hypotonia, dystonia, Marked elevation of GAA levels in plasma, urine and CSF; choreiform movements decreased plasma arginine and urinary creatinine; marked attenuation of creatine peak on MRS; deficiency of GAMT in fibroblasts CDG syndrome Marked psychomotor retardation, generalized hypotonia, Abnormal isoelectric focusing of plasma transferrin dysmorphic features (see Chapter 6) reflecting under-glycosylation BFNC Neonatal-onset seizures, with normal neurological and Mutations in neuronal potassium channel genes (KCNQ2 cognitive outcome or KCNQ3) GEFS+ Febrile seizures occurring over age 6 years Mutations in neuronal sodium channel gene (SCNB1)
Abbreviations: ETC, electron transport chain; NCL, neuronal ceroid-lipofuscinosis; PDH, pyruvate dehydrogenase; MMA, methylmalonic acidemia; THF, tetrahy- drofolate; MELAS, mitochondrial encephalomyopathywith lactic acidosis and stroke-like episodes; GLD, globoid cell leukodystrophy; VLCFA, very long-chain fatty acids; GAMT, guanidinoacetate methyltransferase; GAA, guanidinoacetic acid; MRS, magnetic resonance spectroscopy; CDG, congenital disorder of glycosylation; BFNC, benign familial neonatal convulsions; GEFS+,generalized epilepsy with febrile seizures plus syndrome. 38 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.2 MRI scans of the brain of a patient with juvenile neuronal ceroid-lipofuscinosis. T2-weighted axial MRI scans [TR2800/TE90] through the posterior fossa (a) and basal gan- glia (b)ofa4-year-old child with juvenile neuronal ceroid-lipofuscinosis showing volume loss of both supra- and infra-tentorial structures, most marked in the cerebellum. Abnormal signal is also seen in the internal capsules and internal medullary lamina of the thalamus bilaterally.
to blindness is a prominent and early feature of most of the variants of this group of disorders. It is more likely to be the presenting problem in children over the age of three years, but it is not a feature of adult-onset variants of the disease. Mac- ular degeneration, marked attenuation of retinal blood vessels, peripheral ‘bone spicule’ pigment deposits, and optic atrophy are typical ocular findings in the early-onset variants. Early extinction of the electroretinogram (ERG) is a classical feature of most early-onset variants of NCL. Figure 2.2 shows MRI findings in juvenile NCL. Electron microscopic examination of conjunctival epithelium, skin, peripheral blood leukocytes, or rectal mucosa, shows the presence of typical amor- phous or membranous inclusions (Figure 2.3). The various subtypes of NCL are summarized in Table 2.4. Cherry-red spot-myoclonus syndrome (sialidosis, type I) may present with seizure-like polymyoclonia in later childhood or adolescence with little or no evi- dence of dementia. However, vision is usually impaired, and ophthalmoscopic examination reveals the presence of a prominent cherry-red spot in the macula. The urinary oligosaccharide pattern is abnormal, and demonstrating deficiency of -neuraminidase in cultured fibroblasts confirms the diagnosis. 39 Neurologic syndrome
Figure 2.3 Electron micrograph of conjunctival epithelium showing curvilinear and fingerprint inclu- sions in a patient with neuronal ceroid-lipofuscinosis. The bar represents 1 m. (Courtesy of Dr. Venita Jay, Toronto, Canada.)
Seizures with persistent lactic acidosis may be the first indication of an inher- ited metabolic disorder of mitochondrial energy metabolism, such as pyruvate dehydrogenase (PDH) deficiency or mitochondrial electron transport chain (ETC) defects. The most aggressive clinical variant of this group of disorders is Leigh disease (subacute necrotizing encephalomyelopathy). It is characterized by onset of feeding difficulties and failure to thrive, usually in the first or second year of life. Seizures generally occur on a background of psychomotor retardation then regres- sion, hypotonia, oculomotor abnormalities, recurrent episodes of apnea, ataxic breathing, and tachypnea. The course of the disease is variable. The neurologic deterioration is often punctuated by periods of partial recovery, then acute dete- rioration. In some infants, progression of the disease appears to arrest for periods of up to several months. There is no effective treatment for the disease, and death generally occurs within weeks to a few years after the onset of symptoms. Persistent lactic acidosis is typical of most patients with Leigh disease, regardless of the underlying biochemical lesion. However, sometimes it is difficult to deter- mine whether lactate accumulation is the result of a primary defect in lactic acid metabolism, or simply the normal response to uncontrolled seizure activity. In a small proportion of patients, plasma lactate levels may be normal much of the time. Measurement of cerebrospinal fluid (CSF) lactate levels is helpful in these Table 2.4 Classification of neuronal ceroid lipofuscinoses
Clinical variant Clinical features Anatomic abnormalities Biochemical abnormality Gene
INCL (Santavuori-Haltia Onset in the first year of life of GROD Palmitoyl protein thioesterase CLN1 disease) developmental delay, hypotonia, (PPT) exaggerated startle, followed by psychomotor regression, visual failure, ataxia, dystonia, spasticity, myoclonic seizures; retinal dystrophy; extinction of ERG; cerebral atrophy on MRI LINCL Onset at 2–4 years of seizures, Lysosomal curvilinear bodies Tripeptidyl peptidase 1 (TPP1); CLN1, CLN2 (Jansky-Bielschowsky myoclonus, psychomotor neuronal accumulation of disease) retardation, ataxia, progressive SCMAS visual impairment, early extinction of ERG JNCL (Batten disease or Onset at 4–7 years of progressive visual Vacuolated lymphocytic Lysosomal membrane CLN1,CLN2, Spielmeyer-Vogt- impairment, followed sometimes inclusions on peripheral glycoprotein of uncertain CLN31 Sjogren¨ years later by dysarthria, intellectual smear; intralysosomal function disease) deterioration, seizures, behavioral finger-print bodies and other psychiatric disturbances, myoclonia, ataxia ANCL (Kufs disease) Onset in third decade of progressive Mixed intralysosomal inclusions Unknown Unknown intractable myoclonic epilepsy with of finger-print, curvilinear, dementia, ataxia and pyramidal and and granular osmiophilic extrapyramidal signs, or behavior inclusions disturbances and dementia, ataxia, pyramidal and extrapyramidal signs, suprabulbar deficits. Eyes unaffected. Finnish vLINCL Onset at 4–7 years of age of motor Mixed finger-print, curvilinear, Soluble lysosomal glycoprotein CLN5 clumsiness, then progressive visual and rectilinear2 inclusions of uncertain function; failure, psychomotor deterioration, immunoreactive staining for and later by ataxia, myoclonia and SCMAS seizures. Gypsy/Indian vLINCL Onset at 18 mons-5 years of speech Cytosomal finger-print and Endoplasmic reticulum CLN6 (Lake-Cavanagh delay, then seizures, ataxia, rectilinear inclusions in brain; membrane protein of disease; early juvenile myoclonus, and rapidly progressive finger-print bodies in uncertain function NCL) psychomotor regression, late neurones of myenteric plexus progressive visual impairment. Turkish vLICNL Onset at 3–7 years of progressive visual Cytosomal finger-print, Endoplasmic reticulum CLN8 impairment, delayed speech, rectilinear and curvilinear membrane protein of seizures, cognitive deterioration, inclusions; immunoreactive uncertain function myoclonia, ataxia. staining for SCMAS Northern EPMR (rare) Onset at 5–10 years of generalized CLN8 tonic-clonic seizures, then slowly progressive psychomotor deterioration.
Note: For updated catalogue of mutations, see www.ucl.ac.uk/ncl Abbreviations: GROD, granular osmiophilic deposits; ERG, electroretinogram; SCMAS, subunit c of mitochondrial ATP synthase; EPMR, epilepsy and progressive mental retardation; INCL, infantile neuronal ceroid lipofuscinosis; LINCL, late-infantile neuronal ceroid lipofuscinosis; JNCL, juvenile neuronal lipofuscinosis; vLINCL, variant LINCL; ANCL, adult neuronal ceroid lipofuscinosis, 1 A large deletion involving two exons of CLN3 accounts for over 70% of the mutant alleles associated with this disease. 2 Stacks of short, straight, oligolamellar structures. Source: Table is adapted from Goebel & Wisniewski (2004). 42 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.4 Axial MRI scan and MRS of the brain of a patient with Leigh disease. Panels a, Axial FLAIR MRI scan [TR9000/TE160, TI2200] of a 17-month-old child showing increased signal in periaqueductal gray matter and cerebral peduncles; b, MRS showing prominent lactic acid doublet. 1, choline; 2, creatine; 3, N-acetylaspartate; 4, lactate. 43 Neurologic syndrome
Figure 2.5 Coronal MRI scan of the brain of a child with Leigh disease. 1 Coronal fast spin echo, T2-weighted MRI scan [TR4000/TE63] of the brain of a 4 /2-year-old child showing typical increased signal in the putamina and caudate heads.
situations. CSF lactate levels are not as likely to be spuriously elevated as plasma lactate levels, and they are often elevated in patients with primary disorders of lactic acid metabolism even when plasma levels are normal. Rarely, both plasma and CSF lactates are normal. Imaging studies often show destructive lesions in the brainstem (Figure 2.4) and basal ganglia and thalamus (Figure 2.5). Confirmation of the diagnosis requires biochemical studies on fibroblasts or skeletal muscle (see Chapter 9). Alper’s disease (progressive infantile poliodystrophy) is a clinical syndrome, similar to Leigh disease, characterized by onset in early childhood of psychomo- torretardation, then regression, disturbances of tone, myoclonic or tonic-clonic seizures, ataxia, and episodic tachypnea. The principal differences are the promi- nence in Alpers disease of seizures and cortical blindness, a reflection of the greater involvement of the cerebral cortex, and evidence of hepatocellular dysfunction. This syndrome has been reported in infants with various inborn errors of energy metabolism,particularlyPDHdeficiencyandmitochondrialETCdefects.Persistent lactic acidosis is common, often becoming severe during intercurrent infections. The approach to diagnosis is the same as for Leigh disease. 44 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.6 Axial MRI scan of the brain of a child with MELAS. Axial FLAIR MRI scan [TR9002/TE165] through the basal ganglia of an 8-year-old child show- ing stroke-like lesion in the occipital lobe and junction of the thalamus and internal capsule.
Patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) generally present in middle-to-late childhood with a history of psychomotor delay, growth failure, headaches, vomiting, and seizures. Alternating hemiparesis and visual field defects or blindness, exercise intolerance, and muscle weakness, are also common and prominent features of the disease. During episodes of acute encephalopathy, lactate levels may rise to 10–20 mmol/L. Despite the name given to the disease, plasma lactate levels between episodes of metabolic decompensation are not always elevated. However, CSF lactate levels are generally two to three times above normal. CSF protein concentrations are also increased. Imaging studies typically show patchy cortical abnormalities indicative of ischemic damage (Figure 2.6). These do not always conform to the distribution of major cerebral arteries. Histochemical studies on skeletal muscle biopsies show ragged- redfibers. Biochemical studies on muscle often show deficiency of Complex I or Complexes I and IV of the mitochondrial ETC. A particularly common defect in patients with this disease is a point mutation in mitochondrial tRNALeu(UUR). Endocrinopathies, especially type I diabetes mellitus, are common in patients with MELAS. In fact, diabetes may be the first, and sometimes the only, manifestation of the disease, predating the onset of neurological disturbances by several months or even years. 45 Neurologic syndrome
Measurement of the ratio of lactate to pyruvate (L/P) in plasma or CSF in patients with chronic progressive encephalopathy and lactic acidosis establishes whether lactate accumulation is the result of pyruvate accumulation or accumulation of NADH (see Chapter 3). If the L/P ratio is normal, the accumulation of lactate is the result of a defect in pyruvate metabolism, either pyruvate carboxylase (PC) defi- ciency, type A, or PDH deficiency. Measurement of the enzyme activities in leuko- cytes or fibroblasts will confirm the diagnosis. If the L/P ratio is increased and the 3-hydroxybutyrate-to-acetoacetate ratio is decreased, a diagnosis of PC deficiency, type B, should be considered, though this condition always presents in the newborn period and is associated with the presence of other abnormalities (hyperammone- mia and increased plasma levels of citrulline, lysine and proline) which should suggest the diagnosis. Again, measurement of PC activity in leukocytes or fibrob- lasts confirms the diagnosis. Although mitochondrial ETC defects are sometimes identifiable from studies on cultured skin fibroblasts, confirmation of a diagnosis often requires muscle biopsy with histochemical studies, electron microscopy, and biochemical studies on mitochondrial electron transport in mitochondria isolated fresh from the tissue (see Chapter 9). Leigh disease presenting in early infancy with seizures, severe lactic acidosis, renal tubular dysfunction, and cardiomyopathy has been reported in some infants
with muscle coenzyme Q10 (ubiquinone, CoQ10)deficiency. Although treatment of infants with the disease with CoQ10 may improve the lactic acidosis, the long- term outcome is uniformly poor. By contrast, adults with encephalopathic CoQ10 deficiency, presenting as a similar Leigh-like, acute-on-chronic, encephalopathy,
appear to improve on treatment with high-dose coenzyme Q10. Infants with the classical infantile variants of GM2 gangliosidosis – Tay-Sachs dis- ease and Sandhoff disease – usually present at 6–12 months of age with a history of developmental arrest, hypotonia, visual inattentiveness, markedly exaggerated star- tle reflex, and macrocephaly. Visual failure and seizures occur early and are difficult to control. Fundoscopic examination reveals macular cherry-red spots, which are, in this clinical context, virtually pathognomonic of the disease. CT scans of the brain often show typical abnormalities in the thalamus (Figure 2.7). Although Tay-Sachs mutations are common among Ashkenazi Jews, the incidence of the disease in the Jewish community has dropped dramatically over the past 30 years as a result of car- rier screening, genetic counselling, and prenatal diagnosis. In our own experience, the majority of affected infants seen over the past 20 years have not been Jewish. Tay-Sachs disease is caused by deficiency of -hexosaminidase A. The diagnosis is confirmed by measurement of the enzyme in plasma, leukocytes, or fibroblasts. Sandhoff disease is a panethnic disease that is much more rare than Tay-Sachs dis- ease, though clinically almost indistinguishable from it. Infants with Sandhoff dis- ease often show mild hepatomegaly, some thickening of alveolar ridges, and radio- graphic evidence of very mild dysostosis multiplex in addition to all the features of 46 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.7 Axial CT scan of the brain of an infant with Tay-Sachs disease. Axial CT scan of the brain of a 16-month-old child showing ‘bright thalami’, typical of classical Tay-Sachs disease.
Tay-Sachs disease, including macrocephaly and typical macular cherry-red spots. The disease is caused by deficiency of both -hexosaminidase A and B, which is easily demonstrated in plasma, leukocytes, or fibroblasts. In children with peroxisomal disorders, such as Zellweger syndrome, pseudo- Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum disease, and rhizomelic chondrodysplasia punctata, cerebral cortical disorganization is often prominent, and seizures are a common early manifestation of the disorders. Dysmorphism, severe psychomotor retardation, sensorineural deafness, peripheral neuropathy, pigmentary retinopathy, failure to thrive, and evidence of hepatocel- lular dysfunction, may be absent or subtle compared with the prominence of the seizures, particularly early in the course of the later-onset variants. These conditions are discussed in more detail in Chapter 6.Analysis of very long-chain fatty acids, pipecolic acid, and bile acid intermediates in plasma and plasmalogen concentra- tions in erythrocytes is usually sufficient to establish the diagnosis (see Chapter 9). Early-onset, severe psychomotor retardation, profound generalized hypotonia, variable facial dysmorphism, and intractable seizures are also characteristic of most of the variants of congenital disorders of glycosylation (CDG) syndrome. These, too, are discussed in more detail in Chapter 6. 47 Neurologic syndrome
Figure 2.8 CT scan of the brain of an infant with Canavan disease. 1 Axial CT scan of the brain of a 2 /2-year-old child showing diffuse abnormal attenuation of white matter and subcortical gray structures.
White matter disease (leukodystrophy) In diseases predominantly affecting cerebral white matter, the clinical presentation tends to be dominated by motor difficulties, including gross motor delay, weakness, and incoordination. White matter disease (leukodystrophy) is a common feature of many inherited metabolic disorders presenting with chronic encephalopathy, including many ‘small molecule’ diseases. Therefore, the investigation of any patient presenting with signs of leukodystrophy should routinely include analysis of plasma amino acids and urinary organic acids. The leukodystrophy in patients with Canavan disease is particularly aggressive and typically associated with rapidly developing megalencephaly. Affected infants present in the first few months of life with a history of developmental arrest, irri- tability, hypotonia, and failure to thrive, followed by spasticity and seizures. Imaging studies show severe, diffuse white matter attenuation (Figure 2.8). The disease is caused by deficiency of aspartoacylase, which is associated with accumulation of N-acetylaspartate (NAA) in the CSF, blood, and urine. Diagnosis is suggested by finding increased NAA levels in urine by GC-mass spectrometry, or in the brain by magnetic resonance spectroscopy, and it is confirmed by direct demonstra- tion of the enzyme deficiency in fibroblasts. The incidence of this disease is high 48 A Clinical Guide to Inherited Metabolic Diseases
among Ashkenazi Jews in whom a single mutation (E285A) accounts for 80–85% of the mutant alleles, with only two other mutations accounting for the bulk of the remainder. Classical Alexander disease is a phenocopy of Canavan disease with early onset of developmental arrest, hypotonia, seizures, and marked megalencephaly. Imaging studies show severe white matter disease. The diagnosis is usually made by demon- stration of Rosenthal fibers in the brain at autopsy. The disease has recently been shown to be associated with autosomal dominant mutations in the GFAP gene, coding for glial fibrillary acidic protein. Late-onset variants of Alexander disease are not associated with megalencephaly. Pseudobulbar and bulbar signs dominate the onset of the juvenile-onset variant; patients with adult-onset disease show signs of pyramidal tract and cerebellar dysfunction, mimicking multiple sclerosis, with palatal myoclonus in at least some. Imaging studies show atrophy of the cerebellum, medulla, and spinal cord. Boys with X-linked adrenoleukodystrophy (XL-ALD) generally present in middle childhood with a history of behavior problems (irritability, withdrawal, obsessive- ness) or school failure, followed by the development of gait disturbances, increased muscle tone progressing to spasticity, visual failure, and deafness. Deterioration to aneuro-vegetative state occurs rapidly, though death may be delayed for several years. Some boys present with overt clinical evidence of adrenal insufficiency, such as a history of fatiguability and deep tanning of the skin. All boys with the condition show at least biochemical evidence of adrenal failure sometime in the course of the disease. The CT and MRI changes are so typical that they immediately suggest the diagnosis (Figure 2.9), which is confirmed by measurement of very long-chain fatty acids in plasma. The onset of disease in males who have inherited an ALDP mutation causing XL-ALD may be delayed for several years. Late-onset variants of the disease, called adrenomyeloneuropathy (AMN), are often clinically difficult to distinguish from progressive multiple sclerosis. Progressive spastic paraplegia occurs in a small but significant number of female carriers of XL-ALD. Dementia is late and only very slowly progressive. However, the biochemical abnormalities are the same as in classi- cal juvenile-onset XL-ALD. Curiously, many different clinical variants of the disease may occur in male members of the same family. This makes genetic counselling and the evaluation of any treatment for this disorder particularly difficult. Many patients with late-onset variants of GM2 gangliosidosis present with motor difficulties, such as ataxia, dysarthria, and dystonia, caused by generalized white matter involvement with the disease. Imaging studies show generalized brain atrophy, but posterior fossa structures areusually particularly severely affected (Figure 2.10). Unlike the common infantile-onset variants of the disease (Tay-Sachs disease and Sandhoff disease), macular cherry-red spots are not seen in patients 49 Neurologic syndrome
Figure 2.9 CT and MRI scans of the brain in X-linked adrenoleukodystrophy. Panel a, shows a CT scan of the brain with enhancement done early in the course of the disease in a nine-year-old boy. It shows diffuse white matter attenuation in the peritrigonal area and corpus callosum. The arrows indicate the rim of active demyelination characteristic of the disease. Panel b, shows a T2-weighted MRI scan [TR2800/TE90] done 10 years later. It shows extensive demyelination of the peritrigonal area and corpus callosum extending into subcortical white matter with incomplete sparing of U-fibers.
with late-onset forms of the disease. The diagnosis is confirmed by measurement of -hexosaminidase A and B in plasma, leukocytes, or fibroblasts. The presence of peripheral neuropathy may not be clinically obvious, but it is an important feature of inherited disorders of myelin lipid metabolism, such as metachromatic leukodystrophy and Krabbe globoid cell leukodystrophy. Metachro- matic leukodystrophy (MLD) is caused by deficiency of arylsulfatase A and is char- acterized by accumulation of the myelin lipid, sulfatide, in the brain and peripheral nerve. The clinical presentation in early onset variants of MLD is usually domi- nated by signs of motor difficulties, such as weakness, clumsiness, and stumbling, resembling ataxia. Nerve conduction studies show slowing, and the CSF protein concentration is characteristically elevated. Cognitive functioning is only mini- mally affected at first. The diagnosis is confirmed by measurement of arylsulfatase Ainleukocytes or fibroblasts. Later in the course of the disease, and early in the course of late-onset variants of MLD, cognitive dysfunction is more prominent. 50 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.10 MRI scan of the brain in late-onset GM2 gangliosidosis. Sagittal fast spin echo T2-weighted MRI [TR4000/TE63] of the brain of a 31-year-old woman showing severe volume loss of the cerebellum.
Children with juvenile-onset MLD generally come to attention as a result of dete- riorating school performance, though the presence of motor difficulties and some dysarthria can usually be demonstrated on careful physical examination. In patients with adult-onset MLD, cognitive impairment is commonly accompanied by psy- chiatric disturbances. General physical and routine neurological examinations are often normal, though careful mental state examination will usually show deficits, which in the early stages may be limited to higher cognitive abnormalities, especially executive dysfunction.
Chronic encephalopathy – with non-neural tissue involvement
The pattern of non-neural tissue involvement in patients presenting with chronic encephalopathy is an important clinical clue to the underlying defect. Many of the diseases exhibiting significant non-neurologic involvement are caused by defects in organelle metabolism. Those in which myopathy is particularly prominent are considered in the section on ‘Myopathy’. Hepatosplenomegaly is a prominent feature of many of the lysosomal storage diseases presenting as chronic encephalopathy. In some, such as Hurler disease 51 Neurologic syndrome
(MPS IH), Hunter disease (MPS II), and Sly disease (MPS VII), the non-neurologic manifestations of disease dominate the clinical presentation, and they are discussed in Chapter 6.Incontrast, the hepatosplenomegaly in patients with Sanfilippo dis- ease (MPS III) is rarely very impressive and the radiographic evidence of dysostosis multiplex may be very subtle. Patients with Sanfilippo disease usually present in the second or third year of life with a history of developmental delay, particu- larly affecting speech, and characteristically horrendous behavior problems char- acterized by marked impulsivity, aggressiveness, hyperactivity, stereotypic motor automatisms, and nocturnal restlessness. The behavior problems are sufficiently characteristic to suggest the diagnosis. Thefour biochemically and genetically dis- tinct variants of Sanfilippo disease (MPS IIIA, B, A, and D) are clinically indis- tinguishable from each other. Urinary MPS screening tests are sometimes falsely negative. Thin-layer chromatography of urinary MPS typically shows increased excretion of heparan sulfate. The diagnosis is confirmed by analysis in fibroblasts of each of the four enzymes found deficient in different variants of the disease (see Chapter 9). Infants with acute neuronopathic Gaucher disease (type 2) present in the first few months of life with developmental arrest, hypertonia, neck retraction, strabismus, visual impairment, and major feeding difficulties as a result of inability to swallow. The liver and especially the spleen are typically huge, but bone changes, which are so prominent in many patients with non-neuronopathic Gaucher disease (type 1), do not occur. Storage cells are not seen in the peripheral circulation. However, bone marrow aspirates contain typical Gaucher cells, which are indistinguishable from those seen in non-neuronopathic variants of the disease. The diagnosis is confirmed by demonstrating deficiency of lysosomal -glucosidase (or glucocerebrosidase) in leukocytes or fibroblasts. Most infants with acute neuronopathic Gaucher disease carry at least one L444P mutation. This is a rapidly progressive disease, generally ending in death before age two years. Children with subacute neuronopathic Gaucher disease (type 3) usually present in early or middle childhood with a history of slowly progressive ataxia, dysarthria, and cognitive deterioration, similar is many respects to juvenile MLD. However, unlike children with MLD, children with type 3 Gaucher disease almost always have significant hepatosplenomegaly. Some children with type 3 disease present in early childhood with aggressive visceral disease, with little or no clinical evi- dence of neurologic involvement for some years. Superficially, the disease in these children resembles a very severe form of type 1 Gaucher disease. In fact, some patients with type 3 disease die of hepatic failure before they develop significant neurologic problems. Survivors invariably develop an unusual oculomotor abnor- mality, characterized by vertical looping movements of the eyes on lateral gaze, the first clinical clue to the true nature of the condition, sometimes long before 52 A Clinical Guide to Inherited Metabolic Diseases
the appearance of other neurologic abnormalities. Biochemically, patients with this disease are indistinguishable from patients with severe non-neuronopathic Gaucher disease (see Chapter 6). Mutation analysis is of some help: most patients with type 3 Gaucher disease, like those with type 2 disease have at least one L444P allele. In contrast, the presence of the common N370S alleles is virtu- ally unknown in patients with neuronopathic Gaucher disease, either type 2 or type 3. Infants with Niemann-Pick disease (NPD), especially type A, also present in the first few months of life with typically massive enlargement of the liver and spleen causing marked protuberance of the abdomen. However, neurologic involvement with the disease occurs later and is more slowly progressive that in the acute neu- ronopathic variant of Gaucher disease. Feeding problems commonly cause severe failure to thrive, and pulmonary involvement often causes chronic respiratory problems. Liver function tests may be mildly abnormal. Skeletal radiographs are usually normal. However, radiographs of the chest commonly show diffuse retic- ular infiltrations of the lungs. Bone marrow smears usually show the presence of foamy storage histiocytes, whichare typical though not specific for the dis- ease. The disease is caused by deficiency of lysosomal acid sphingomyelinase. The diagnosis of the disease is confirmed by measuring the enzyme in leukocytes or fibroblasts. NPD type C (NPD-C) may present in infancy or early childhood as hepatic syndrome (see Chapter 4), or as a progressive neurodegenerative condition with relatively little evidence of visceral involvement. Presentation as neurologic disease is usually in early to middle childhood with progressive gait disturbance, dysarthria, emotional lability, and developmental arrest, then regression. The liver and spleen are usually enlarged, and liver function test is often mildly abnormal. One of the characteristic features of the condition is early-onset supranuclear gaze palsy, man- ifested as impaired vertical saccadic eye movements. Bone marrow aspirates usu- ally show the presence of foamy histiocytes and ‘sea-blue’ histiocytes. The basic biochemical defect in NPD-C is incompletely understood. It appears to involve some disturbance in the intracellular processing of cholesterol. The esterification of cholesterol by cultured skin fibroblasts is typically impaired. The cells also show strong staining with filipin as a result of cholesterol accumulation in the cells. Con- firming the diagnosis of NPD-C by laboratory studies is often difficult because the diagnostic abnormalities are secondary manifestations of the primary metabolic defect. Mutation analysis is particularly important in this group of diseases because it is the only reliable method for the identification of carriers of the disease, and it greatly facilitates prenatal diagnosis. Most cases of NPD-C are caused by mutations in the NPC1 gene; some are caused by NPC2 mutations. 53 Neurologic syndrome
GM1 gangliosidosis and sialidosis, along with other glycoproteinoses, are reviewed in Chapter 6.Both conditions may present in early infancy with severe, rapidly progressive, neurovisceral storage disease associated with dysmorphic facial featuresresemblingHurlerdisease,butwithoutcornealclouding,oftenwithcherry- redmacular spots, and radiographic evidence of bone involvement. Patients with later-onset variants have little or no evidence of non-neurologic disease. As a rule, they present with gait disturbances, dysarthria, and psychomotor retardation. Spasticity and seizures follow. Analysis of urinary oligosaccharides is abnormal in both conditions. Definitive diagnosis requires demonstration of deficiency of -galactosidase in plasma, leukocytes, or fibroblasts in the case of GM1 gangliosi- dosis. Confirmation of the diagnosis of sialidosis requires the demonstration of -neuraminidase deficiency in cultured fibroblasts. Fucosidosis and mannosidosis commonly present as chronic encephalopathy with developmental delay. Although both, especially fucosidosis, are associated with the development of ‘storage facies’ as they grow older, psychomotor retardation is often the only clinical complaint at initial presentation. Radiographic evidence of mild dysostosis multiplex is usually present, though subtle and often overlooked. Vacuolated mononuclear cells are often found in peripheral blood smears. Patients with mannosidosis characteristically develop sensorineural hearing loss early in the course of their disease. Angiokeratomata, indistinguishable from those seen in patients with Fabry disease, are a typical feature of fucosidosis. The non-neurologic features of homocystinuria and Menkes disease clearly set them apart from other inherited metabolic diseases in which chronic encephalopathy is a prominent aspect of the clinical presentation. They are dis- cussed in Chapter 6.
Acute encephalopathy
Acute encephalopathy, regardless of the cause, is a medical emergency. In addi- tion to being a common manifestation of a variety of acquired medical or surgical conditions, it is a presenting feature of a number of inherited metabolic diseases, particularly in young children (Figure 2.11). Deterioration of consciousness occur- ring as a result of inherited metabolic disease: r often occurs with little warning in a previously healthy infant or child; r may be missed because the early signs may be mistaken as a behavior disorders; r often progresses rapidly, may fluctuate markedly; r usually shows no focal neurologic deficits. The earliest signs of encephalopathy may be no more obvious than excessive drowsiness, unusual behavior, or some unsteadiness of gait. Acute or intermittent 54 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.11 Summary of major causes of acute encephalopathy. Abbreviations: UCED, urea cycle enzyme defects; MSUD, maple syrup urine disease; NKHG, nonketotic hyperglycinemia; ETC, electron transport chain.
Table 2.5 Causes of metabolic acute encephalopathy to be considered at various ages
Age
Condition Newborn Early childhood Later childhood
Urea cycle defects ++++ + (girls with OTC) (+) NKHG ++++ 00 Organic acidopathies ++++ + (+) MSUD ++++ ++ ++ FAOD + ++++ ? Reye syndrome 0 ++ +++ Drug ingestion + (maternal) +++ +++
Abbreviations: NKHG, nonketotic hyperglycinemia; MSUD, maple syrup urine disease; FAOD, fatty acid oxidation defects; OTC, ornithine transcarbamoylase deficiency.
ataxia is a common sign of acute encephalopathy in older children with inborn errors of metabolism. A history of recurrent attacks of unsteadiness of gait or ataxia, especially when associated with vomiting or deterioration of consciousness, should be considered a strong indication for investigation of a possible inherited metabolic disease. The progression to stupor and coma is often irregular, with periods of apparent lucidity alternating with periods of obtundation. Failure to recognize the inherent instability of the situation, and to monitor clinical neurologic vital signs closely, may end in disaster. The likely causes of acute metabolic encephalopathy are age- dependent (Table 2.5). 55 Neurologic syndrome
Table 2.6 Initial investigation of acute encephalopathy
Blood gases and electrolytes (calculate anion gap), blood glucose Urinalysis, including tests for ketones and reducing substances Liver function tests Blood ammonium Plasma lactate Urinary organic acids (15 ml urine, no preservative, stored frozen) Plasma amino acid analysis, quantitative Plasma carnitine and acylcarnitines, including tandem MSMS
Table 2.7 Differential diagnosis of inherited metabolic diseases presenting as acute encephalopathy
UCED MSUD OAuria FAOD ETC defects
Metabolic acidosis 0 ± +++ ± ++ Plasma glucose N N or ↓↓↓ ↓↓↓ N Urinary ketones N ↑↑ ↑↑ 00 Plasma ammonium ↑↑↑ N ↑↑ ↑ N Plasma lactate N N ↑±↑↑↑ Liver function N N N ↑↑ N Plasma carnitine N N ↓↓↓ ↓↓ N Plasma amino acids Abnormal ↑ BCAA ↑ glycine ↑ alanine Urinary organic acids N Abnormal Abnormal Abnormal N
Abbreviations: UCED, urea cycle enzyme defect; MSUD, maple syrup urine disease; OAuria, organic aciduria; FAOD, fatty acid oxidation defect; ETC, mitochondrial electron transport chain; BCAA, branched-chain amino acids; ↑ elevated; ↓ decreased; +,present; ±,variably present; N, normal; 0, not present.
Initial investigation Because of the importance of identifying treatable inherited metabolic diseases early, initial investigation of any patient presenting stuporous or obtunded must not be delayed (Table 2.6). Asummary of the results of initial laboratory studies in various inborn errors of metabolism presenting as acute encephalopathy is shown in Table 2.7.
Hyperammonemia The plasma or blood ammonium should be measured immediately, along with the plasma glucose and electrolytes, in any child presenting with acute or sub- acute encephalopathy of obscure etiology. However, the interpretation of the results requires additional information. Plasma ammonium levels are often elevated 56 A Clinical Guide to Inherited Metabolic Diseases
in patients with severe hepatocellular dysfunction, regardless of the cause, includ- ing viral infections, intoxications, or some inborn errors of metabolism. Inherited metabolic diseases presenting with hyperammonemia due to liver failure are dis- cussed in Chapter 4.Apart from marked elevation of plasma ammonium levels, liver function tests in patients with primary disorders of urea biosynthesis are usu- ally near normal. Ornithine transcarbamoylase (OTC) deficiency is an exception to this generalization. Transaminases in patients with the disease are often mildly to moderately elevated, but the hyperammonemia is generally much more severe than can be explained by the degree of hepatocellular dysfunction, as reflected by the transaminases and other tests of liver cell damage. The investigation and diagnosis of possible urea cycle enzyme defects (UCED) is facilitated by reference to a simplified diagram showing the main elements of ammonium metabolism (Figure 2.12). The key features of the metabolism of waste nitrogen are: r The process is divided between two sets of reactions, one set in the cytosol, the other inside mitochondria. r The first reaction, the carbamoylphosphate synthase I (CPS I)-catalyzed conden- sation of ammonium with bicarbonate to form carbamoylphosphate, requires the presence of N-acetylglutamate (NAG), an obligatory effector, not a substrate, for the reaction. r One of the two waste nitrogen atoms that become part of urea is derived from the non-essential amino acid, aspartate. Aspartate is produced by transamination of oxaloacetateinareactioncatalyzedbyliverandmuscleaspartateaminotransferase (AST). r The entire process is highly dependent on an adequate supply of intramitochon- drial ornithine. Ornithine is a five-carbon amino acid analogue of the essential amino acid, lysine. It is formed from arginine by a reaction catalyzed by arginase. The concen- tration of the amino acid is directly related to the availability and metabolism of arginine. Ornithine is not incorporated into body protein. Transport into mito- chondria is facilitated by a specific carrier system. Ornithine is a precursor in the biosynthesis of spermine and putrescine, as well as the amino acids, glutamate and proline. Intramitochondrial ornithine condenses with carbamoylphosphate in a reaction catalyzed by OTC, which is coded by a gene on the short arm of the X chromosome. The product, citrulline, diffuses out of the mitochondrion into the cytosol where it condenseswithaspartatetoformargininosuccinicacid(ASA)inareactioncatalyzed by ASA synthetase. ASA is cleaved to produce arginine and fumarate in a reaction catalyzed by ASA lyase. The renal clearance of ASA is extremely high. 57 Neurologic syndrome
Figure 2.12 Summary of normal ammonium metabolism. The various enzyme of transport systems involved are: 1, N-acetylglutamate synthetase (NAGS); 2, carbamoylphosphate synthetase I (CPS I); 3, ornithine transcarbamoylase (OTC); 4, argininosuccinic acid synthetase (ASA synthetase); 5, argininosuccinic acid lyase (ASA lyase); 6, arginase; 7, mitochondrial ornithine transport system; 8, ornithine aminotrans- ferase. Other abbreviations: GLU, glutamate; CIT, citrulline; ARG, arginine; ORN, ornithine; P5C, 1-pyrroline-5-carboxylic acid; PRO, proline. The figure shows how one of the waste
nitrogen atoms excreted as urea is derived from ammonia (NH3); the other comes from the amino acid, aspartate.
Awidely used algorithmic approach to the differential diagnosis of hyperam- monemic encephalopathy is presented in Figure 2.13.Thepresence of moderate to severe metabolic acidosis indicates that the hyperammonemia is a manifesta- tion of an inherited disturbance of organic acid metabolism, which is discussed in Chapter 3. Urea cycle enzyme defects presenting as acute hyperammonemic encephalopa- thy, whether in early infancy or later in childhood, are clinically indistinguishable from each other. The most important diagnostic information, after ammonium determination, liver function tests, and analysis of blood gases and plasma glucose 58 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.13 An approach to the diagnosis of hyperammonemia in older children. Abbreviations: OA, organic acids; ASAS, argininosuccinic acid synthetase; ASAL, argini- nosuccinic acid lyase; CPS/NAGS, carbamoylphosphate synthetase I or N-acetylglutamate synthetase; HHH, hyperammonemia-hyperornithinemia-homocitrullinemia syndrome; OTC, ornithine transcarbamoylase; LPI, lysinuric protein intolerance. 59 Neurologic syndrome
and electrolytes, is quantitative analysis of plasma amino acids. The reliance on semi-quantitative or screening tests to measure amino acid levels is a common error in the investigation of inherited metabolic diseases. In the investigation of a patient with hyperammonemia, these are particularly inappropriate. Low concentrations of certain amino acids are as important as excesses in the differential diagnosis of hyperammonemia, and subnormal amino acid levels cannot be detected by any of the qualitative or semi-quantitative amino acid screening tests (see Chapter 9). The analytical chemist should be alerted to the need to identify low levels of amino acids, as well as excesses. The concentration of citrulline is central to the interpretation of the results of amino acid analyses. If the citrulline concentration is markedly elevated, the child has citrullinemia as a result of ASA synthetase deficiency. Citrulline levels that are extremely low suggest the presence of a defect in citrulline biosynthesis, the result of deficiency of OTC, CPS I, or NAG synthetase (NAGS). OTCdeficiency is transmitted as an X-linked recessive condition. Affected boys usually present in the newborn period with severe, usually life-threatening, hyper- ammonemia (see Chapter 7). Symptomatic carrier girls generally present later in childhood with an antecedent history of feeding problems, failure to thrive, inter- mittent ataxia, or intermittent encephalopathy. Unfortunately, the diagnosis in symptomatic girls is often missed until the patients present with acute encephalopa- thy, commonly resulting in irreparable brain damage. Recently, we have encoun- teredasmall number of adults with variants of OTC deficiency which presented in the third or fourth decades of life as sudden onset of progressive hyperammonemic encephalopathy, often progressing rapidly to coma and death, in previously com- pletely healthy individuals. Decompensation in some cases was clearly associated with some intercurrent infection or major surgery; in others, no cause could be identified. Deficiency of OTC causes accumulation of carbamoylphosphate and ammo- nium; diffusion of the excess carbamoylphosphate into the cytosol results in over- production of pyrimidines and the pyrimidine intermediates-orotic acid and oroti- dine. OTC is differentiated from CPS I and NAGS deficiencies by the demonstration of increased concentrations of orotic acid and orotidine in the urine. Citrulline levels that are normal or only moderately elevated are generally an indication of argininosuccinic aciduria, the result of argininosuccinic acid lyase deficiency, or argininemia, caused by arginase deficiency. However, argininemia almost never presents as acute encephalopathy. Instead, patients with this disor- der tend to have only mild to moderate elevations of plasma ammonium, and they present clinically later in infancy and early childhood with ‘cerebral palsy’. The elevation of plasma arginine is generally sufficiently specific to make the diagnosis. 60 A Clinical Guide to Inherited Metabolic Diseases
Argininosuccinicaciduriaischaracterizedbysubnormalargininelevelsinplasma and the presence of markedly increased amounts of the amino acid, argininosuc- cinate, in plasma and urine. The renal clearance of this amino acid is very high, and the concentrations in urine are generally very high compared with the levels in plasma. However the demonstration of the presence of increased concentrations of argininosuccinate in plasma requires no more analysis than has already been done for the usual quantitative analysis of plasma amino acids. Lysinuric protein intolerance (LPI) may also present in later infancy or early childhood as hyperammonemic encephalopathy. This is a protean metabolic dis- order, which may present as growth retardation and hepatomegaly, hematologic abnormalities, pulmonary disease, or renal disease. It is caused by a generalized hereditary defect in dibasic amino acid transport. Plasma arginine, ornithine, and lysine levels are typically markedly subnormal. At the same time, quantitative analy- sis of urinary amino acids shows marked increases in the excretion of the same com- pounds. Intracellular ornithine deficiency causes accumulation of carbamoylphos- phate and ammonium resulting in increased urinary orotic acid and orotidine excretion. Hyperammonemia-hyperornithinemia-homocitrullinemia (HHH) syndrome is another disorder of ammonium metabolism caused by a defect in amino acid trans- port. In this case, the transport of ornithine into mitochondria is defective, resulting in intramitochondrial ornithine deficiency. Paradoxically, plasma ornithine levels are markedly elevated in this condition. However, intramitochondrial ornithine deficiency causes accumulation of carbamoylphosphate and ammonium, and in the same manner as in LPI, this causes carbamoylphosphate accumulation result- ing in increased urinary orotic acid and orotidine concentrations. Citrin deficiency, or adult-onset type II citrullinemia (CTLN2), is an autosomal recessive disease characterized by the sudden onset, often precipitated by drug or alcohol ingestion or infection, of hyperammonemia and acute organic brain syndrome – disorientation, delirium, delusions, stupor – often progressing rapidly to death. The disease is caused by mutations of the SLC25A13 gene, a mitochondrial aspartate-glutamate carrier protein. The hyperammonemia appears to be the result of cytosolic aspartate deficiency, caused by the defect in aspartate transport from inside mitochondria into the cytosol. Plasma citrulline levels are characteristically elevated, and plasma aspartate concentrations are typically low. Argininosuccinic acid synthase activity is impaired, partly because of the deficiency of aspartate, along with an absolute deficiency of the enzyme activity in liver by some currently unknown mechanism. The importance of cytosolic aspartate in urea biosynthesis is shown in Figure 2.12. Unlike patients with other urea cycle enzyme defects, who have an aversion to high protein foods and do better on high-carbohydrate diets, patients with CTLN2 61 Neurologic syndrome
report a preference for high-protein foods and dislike carbohydrates. Most are very thin. The prognosis is generally poor, though the response to liver transplantation is excellent. Almost all known cases have been Japanese.
Leucine encephalopathy (maple syrup urine disease – MSUD) This usually presents in the newborn period as an acute encephalopathy, initially without metabolic acidosis (Chapter 7). Milder variants of the disease may present at any age during childhood. Acute encephalopathy without hyperammonemia or significant metabolic acidosis, on a background of chronic failure to thrive and mild to moderate psychomotor retardation, is typical of MSUD. Decompensation is usu- ally heralded by drowsiness, anorexia, and vomiting. The odor widely described as resembling the aroma of maple syrup is more like the smell of burnt sugar. The urine typically tests positive for ketones. Testing the urine for the presence of -ketoacids by addition of DNPH (dinitrophenylhydrazine) reagent produces a strongly pos- itive reaction. Plasma ammonium levels are characteristically normal. The course of subsequent deterioration is often highly irregular with periods of lucidity alter- nating with stupor, progressing to coma. Signs of intracranial hypertension (pos- turing, dilated and sluggish pupils, periodic breathing) are an indication that the situation is grave and the chances of recovery, even with aggressive treatment, are seriously compromised. Quantitative analysis of plasma amino acids is the most rapid and reliable method to confirm the diagnosis, and it should be done with- out delay. Marked elevations of leucine, isoleucine, and valine, and the presence of alloisoleucine, are diagnostic of MSUD. Modest increases in the branched-chain amino acids are common in children during short-term starvation and should not be confused with mild variants of MSUD. Analysis of urinary organic acids as oxime derivatives shows the presence of a number of branched-chain -ketoacids. However, this generally takes longer than plasma amino acid analysis and does not add much to the diagnosis of MSUD.
Reye-like acute encephalopathy (fatty acid oxidation defects) Acute encephalopathy resembling Reye syndrome is a common presenting feature of the primary inherited disorders of fatty acid oxidation. The commonest of these is medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. Affected children are usually completely well until they present, usually in the first year or two of life, with what may appear initially to be nothing more sinister than ‘stomach flu’, with anorexia, vomiting, drowsiness, and lethargy. The fact that metabolic decompensation is usually precipitated by intercurrent illness, generally associated with poor feeding, often obscures the nature of the underlying metabolic disorder. Drowsiness and lethargy progress rapidly to stupor and coma, hepatomegaly with evidence of hepatocellular dysfunction, hypotonia, hypoketotic hypoglycemia (see 62 A Clinical Guide to Inherited Metabolic Diseases
Chapter 4), and mild to moderate hyperammonemia. Hypoglycemic seizures may be the first serious sign of the disease. Although it is often clinically indistinguishable from Reye syndrome, onset in the first two years of life, a positive family history, and recurrence of acute metabolic decompensation during trivial intercurrent illnesses or fasting, are features peculiar to MCAD deficiency and other fatty acid oxidation defects. Acute encephalopathy may also occur in the absence of hypoglycemia or signifi- cant hepatocellular dysfunction, suggesting that accumulation of fatty acid oxida- tion intermediates plays some role in its pathogenesis. Sudden unexpected death is tragically common in infants with unrecognized MCAD deficiency, perhaps as aresult of cardiac arrhythmias caused by accumulated fatty acid oxidation inter- mediates (see Chapter 5). Successful management of this condition rests on a high index of suspicion coupled with early treatment with glucose while awaiting the results of definitive laboratory tests. Analysis of urinary organic acids during acute metabolic decompensation char- acteristically shows the presence of large amounts of C-6 to C-10 dicarboxylic acids (adipic, suberic, and sebacic acids), the ( -1)-hydroxy derivatives of hexanoic and octanoic acids, but little or no ketones (see Chapter 4). Urinary organic acid analysis when the child is clinically well may be completely normal. However, analysis of acylcarnitines and acylglycines generally shows the presence of octanoylcarnitine, and hexanoylglycine and phenylpropionylglycine, respectively, in urine. Analysis of acylcarnitines in plasma or dried blood spots by tandem MSMS shows accumu- lation of octanoylcarnitine, even when the child is clinically well. Tandem MSMS is being employed by several centers as a method for screening newborns for fatty acid oxidations defects (see Chapter 8). MCAD deficiency is an inherited metabolic disorder in which specific mutation analysisisparticularlyusefulformakingadiagnosis:onemutation,K329E,accounts for over 90% of all MCAD deficiency-associated alleles discovered to date. Testing for the presence of this mutation provides helpful confirmatory evidence for this disease. Other inborn errors of fatty acid oxidation are rare compared with MCAD deficiency. Systemic carnitine deficiency and long-chain acyl-CoA dehydrogenase (LCAD) deficiency may present with a Reye-like acute encephalopathy, but the evi- dence of skeletal muscle involvement is usually more obvious, and cardiomyopathy, which is never seen in MCAD deficiency, is generally prominent. Similarly, short- chain acyl-CoA dehydrogenase (SCAD) deficiency is very rare, and may present as encephalopathy with metabolic acidosis in the newborn period (see Chapter 7). Rarely, carnitine palmitoyltransferase II (CPT II) deficiency, which is usually asso- ciated with a myopathy presenting in later childhood, may present in infancy with recurrent Reye-like acute encephalopathy clinically indistinguishable from MCAD 63 Neurologic syndrome
deficiency. A summary of the urinary organic acid abnormalities in these conditions is shown in Table 2.8.
Acute encephalopathy with metabolic acidosis The inherited organic acidopathies commonly present as acute encephalopathy, the presence of severe metabolic acidosis is usually recognized early in the management of the problem. Detailed treatment of this particular group of disorders is presented in Chapter 3 and Chapter 7.
Hypoglycemia Although conditions like glycogen storage disease, type I (GSD I), often present in infancy with alteration of consciousness, sometimes progressing rapidly to coma and to seizures, the presence of hypoglycemia generally directs the investigation (see Chapter 4).
Stroke
Stroke is a well-recognized result of some inherited metabolic diseases, such as familial hypercholesterolemia and homocystinuria. Over the past few years, an increasing number of inborn errors of metabolism has been reported to be associ- ated with stroke or stroke-like episodes (Table 2.9). Stroke in infancy and childhood is more likely to be the result of inherited defects in metabolism than that occurring in adults. It is generally, though not always, associated with other abnormalities, such as metabolic acidosis, psychomotor retardation, or failure to thrive. In some cases, it is may be the presenting problem. Episodic unilateral migraine headache associated with hemiplegia, aphasia, hemianopia, or facial paresthesia, lasting from hours to days, suggests the pos- sibility of familial hemiplegic migraine (FHM) caused by mutations in one of the calcium channel genes (CACNA1A).
Movement disorder
Extrapyramidal movement disorders in patients with inborn errors of metabolism are almost always associated with neurologic signs referable to other parts of the nervous system (Table 2.10). Unsteadiness of gait, particularly in children, which may be a manifestation of immaturity or muscle weakness, is a particularly common finding in inherited metabolic diseases.
Ataxia One productive approach to the diagnosis in patients in whom ataxia is a prominent is to determine early on whether the problem is static or progressive, and if it is Table 2.8 Organic acid abnormalities in the hereditary fatty acid oxidation defects
MCAD deficiency SCAD deficiency LCAD deficiency LCHAD deficiency ETF/ETF-DH deficiency
5-hydroxyhexanoate ethylmalonate adipate adipate 3-hydroxybutyrate adipate methylsuccinate suberate 3-hydroxyadipate glutarate suberate octenedioate suberate ethylmalonate octenedioate n-butyrylglycine decenedioate octenedioate methylsuccinate 7-hydroxyoctanoate dodecanedioate 3-hydroxysuberate sebacate tetradecanedioate sebacate n-butyrylglycine decenedioate decenedioate isobutyrylglycine 3-hydroxysebacate 3-hydroxysebacate 2-methylbutyrylglycine dodecanedioate isovalerylglycine hexanoylglycine 3-hydroxydodecanedioate hexanoylglycine suberylglycine 3-hydroxydodecenedioate phenylpropionylglycine 3-hydroxytetradecanedioate 3-hydroxytetradecenedioate octanoylcarnitine
Abbreviations: MCAD, medium-chain acyl-CoA dehydrogenase; SCAD, short-chain acyl-CoA dehydrogenase; LCAD, long-chain acyl-CoA dehydrogenase; LCHAD, long-chain 3-hydroxyacyl-CoA dehydrogenase; ETF/ETF-DH, electron transfer flavoprotein/electron transfer flavoprotein dehydrogenase. Bold type indicates the presence of the compound is particularly characteristic of the disease. 65 Neurologic syndrome
Table 2.9 Inherited metabolic diseases associated with strokes or stroke-like episodes
Disease Diagnosis
Homocystinuria, including cystathionine Elevated plasma homocysteine and demonstration of -synthase deficiency, MTHFR deficiency specific enzyme defect and cobalamin defects Fabry disease Deficiency of plasma or leukocyte -galactosidase A activity (males) or demonstration of disease-causing mutation in GLA gene (males or females) Organic acidopathies Abnormalities of urinary organic acids or plasma Methylmalonic acidemia acylcarnitines and demonstration of specific enzyme Propionic acidemia defect Isovaleric acidemia Glutaric aciduria, types I and II Ornithine transcarbamoylase (OTC) deficiency Hyperammonemia, elevated urinary orotic acid, demonstration of mutations in OTC gene MELAS Persistent lactic acidosis, typical changes on MRI of the brain, demonstration of disease-associated mtDNA mutations (see Table 9.14) CDG type Ia Abnormal isoelectric focusing pattern of plasma transferrin Familial hemiplegic migraine Demonstration of mutations in P/Q-type calcium channel gene (CACNA1A)
Abbreviations: MTHFR, methylene tetrahydrofolate reductase; MELAS, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; CDG, congenital disorder of glycosylation.
progressive, whether the course is a relentless one, or if it is intermittent. Early- onset, static ataxia is almost never caused by an inherited metabolic disease. It is more likely to be traceable to a congenital malformation of the brain or brain injury sustained early in life. In childhood, generalized muscle weakness may present as unsteadiness during walking or running, resembling ataxia caused by a primary extrapyramidal disorder. In any child presenting with a history of recurrent acute ataxia separated by periods free of neurologic abnormalities, the possibility of an inherited metabolic disease should be considered. Recurrent episodes of ataxia are a common mani- festation of metabolic decompensation in patients with ‘small molecule’ disorders, such as urea cycle enzyme defects, organic acidopathies, and the mild or intermit- tent variants of MSUD. It may be the only clinical manifestation of various ‘chan- nelopathies’ or of mild pyruvate dehydrogenase deficiency (PDH) deficiency. Mild Table 2.10 Inherited metabolic disease in which movement disorders are prominent
Disease Other clinical features Diagnosis
Intermittent ataxia UCED (e.g., OTC deficiency, CPS I Encephalopathy, dietary protein intolerance, Plasma amino acid abnormalities and deficiency of deficiency, ASAuria, etc.) hyperammonemia specific urea cycle enzymes Organic acidopathies (e.g., MMA, PA, Metabolic acidosis, hyperammonemia Marked excretion of organic acid intermediates in the IVA) urine PDH deficiency (mild) Lactic acidosis Deficiency of PDH in leukocytes and fibroblasts Episodic ataxia, type 1 (EA1) Brief attacks of cerebellar ataxia, with later development of Mutations in potassium channel gene, KCNA1. myokymia of facial, arm and hand muscles, precipitated by sudden changes in posture or stress Episodic ataxia type 2 (EA2) Attacks of headache and ataxia, vertigo, dysarthria, and Mutations in P/Q-type calcium channel genes nystagmus (CACNA1A)
Progressive ataxia Friedreich ataxia Progressive gait ataxia, pes cavus, kyphoscoliosis, sensory Expansion of GAA repeats in frataxin gene (FRDA) neuropathy, dysarthria, nystagmus, sensorineural deafness, absent deep tendon reflexes, extensor plantar reflexes, cardiomyopathy, diabetes mellitus in some AVED Onset in first or second decade of progressive ataxia, visual Marked decrease of plasma -tocopherol levels impairment with retinitis pigmentosa (in some), head titubation, flexor plantar reflexes Abetalipoproteinemia Steatorrhea, anemia, acanthocytosis, psychomotor Marked deficiency of apolipoprotein B in plasma retardation, retinitis pigmentosa Infantile NCL (Santavuori-Haltia Psychomotor retardation, myoclonic seizures, blindness, Typical lysosomal inclusions seen on electron syndrome) early flattening of the EEG microscopic examination of skin, leukocytes, or conjunctival epithelium Mitochondrial ETC defects (e.g., KSS, Lactic acidosis, small stature, retinal degeneration, Defects in mitochondrial ETC in fibroblasts or skeletal MERRF) psychomotor retardation, seizures, myopathy, muscle; mitochondrial mutation analysis (see sensorineural deafness Chapter 9) Late-onset galactosialidosis Myoclonus, seizures, corneal clouding, cherry-red spots, Deficiency of -galactosidase and -neuraminidase in mental retardation fibroblasts Late-onset GM2 gangliosidosis Muscle wasting, dysarthria, exaggerated deep tendon reflexes, Deficiency of -hexosaminidase in serum, leukocytes or psychosis fibroblasts Late-onset MLD Muscle wasting, dysarthria, peripheral neuropathy Deficiency of arylsulfatase A in leukocytes or fibroblasts Late-onset Krabbe GLD Spasticity, visual failure Deficiency of galactocerebrosidase in leukocytes or fibroblasts Refsum disease Retinitis pigmentosa, peripheral neuropathy, sensorineural Elevated phytanic acid levels in plasma; defect in deafness, cataracts, ichthyosis phytanic acid oxidation in fibroblasts Niemann-Pick disease, type C Hepatosplenomegaly, supranuclear vertical gaze palsy, Defect in cholesterol esterification in fibroblasts; NPC1 sea-blue histiocytes in bone marrow mutation analysis Hartnup disease Pellagra-like skin rash Massive neutral, monoamino-monocarboyxlic aminoaciduria l-2-Hydroxyglutaric aciduria Psychomotor retardation, choreoathetosis ± seizures Marked increase in levels of l-2-hydroxyglutaric acid in urine Mevalonic aciduria Psychomotor retardation, facial dysmorphism, failure to Increased mevalonic acid in urine on organic acid thrive, recurrent fever, diarrhea, arthralgia, edema, skin analysis rash Spinocerebellar ataxia type 6 Late-onset, slowly progressive cerebellar ataxia Demonstration of mutations in P/Q-type calcium channel genes (CACNA1A)
Dystonia/choreoathetosis Glutaric aciduria, type I Psychomotor retardation, episodes of acute encephalopathy Marked increase in excretion of glutaric and with metabolic acidosis 3-hydroxyglutaric acids in urine; deficiency of glutaryl-CoA dehydrogenase in fibroblasts Lesch-Nyhan disease Psychomotor retardation, self-mutilatory behavior Increased uric acid levels in plasma and urine; HPRT deficiency in leukocytes or fibroblasts (cont.) Table 2.10 (cont.)
Disease Other clinical features Diagnosis
TPI deficiency Chronic hemolytic anemia, susceptibility to infection, Deficiency of TPI in erythrocytes cardiomyopathy, death in early childhood 4-Hydroxybutyric aciduria Psychomotor retardation, oculomotor abnormalities, Massive excretion of 4-hydroxybutyric acid in urine, choreoathetosis decreasing with age Segawa syndrome Dystonia of extremities growing worse during the course of Dramatic response to treatment with L-dopa; the day, normal intellect GTP-cyclohydrolase assay in fibroblasts
Parkinsonism Wilson disease Dementia, psychiatric problems, hepatocellular dysfunction Marked decrease of plasma ceruloplasmin and copper (Chapter 4), Kayser-Fleischer rings levels and increased urinary excretion of copper Tyrosine hydroxylase deficiency Onset in infancy or generalized rigidity, ptosis, drooling, CSF neurotransmitter analysis: ↓HVA, normal 5HIAA, truncal hypotonia, marked tremor in upper extremities, ↓MHPG, normal 3OMD, normal BH4, normal BH2, occasional myoclonic jerks. Reduced deep tendon reflexes. normal Neopterin Normal MRI and CT scans Aromatic amino acid decarboxylase Early-onset bradycardia, temperature instability, irritability CSF neurotransmitter analysis: ↓HVA, ↓5HIAA, deficiency and poor feeding. Generalized hypotonia with paroxysmal ↓MHPG, ↑3OMD, normal BH4, normal BH2, extension of arms and legs accompanied by eye-rolling, normal Neopterin evolving into an extrapyramidal movement disorder with oculogyric crises and convergence spasm.
Abbreviations: MLD, metachromatic leukodystrophy; ETC, electron transport chain; KSS, Kearns-Sayre syndrome; MERRF, myoclonic epilepsy and ragged-red fiber disease; AVED, ataxia with vitamin E deficiency; NCL, neuronal ceroid-lipofuscinosis; UCED, urea cycle enzyme defects; CPS I, carbamoylphosphate synthetase I; ASAuria, argininosuccinic aciduria; PDH, pyruvate dehydrogenase; MMA, methylmalonic acidemia; PA, propionic acidemia; IVA, isovaleric acidemia; HPRT, hypox- anthine phosphoribosyltransferase; TPI, triose phosphate isomerase; GLD, globoid cell leukodystrophy; HVA, homovanillic acid; 5HIAA, 5-hydroxyindoleacetic acid; MHPG, 3-methoxy-4-hydroxyphenylglycol; 3OMD, 3-O-methyldopa; BH4, tetrahydrobiopterin; BH2, 7,8-dihydrobiopterin. 69 Neurologic syndrome
PDH deficiency, presenting as recurrent ataxia, occurs almost exclusively in young boys, and the episodes of unsteadiness are usually, though not always, associated with lactic acidosis. Intermittent ataxia may be the presenting clinical problem in young children with acute intermittent muscle weakness, such as occurs in the early-onset ion channelopathies (see below). Progressive ataxia is often the presenting problem in many of the late-infantile or juvenile-onset variants of organelle diseases, such as late-infantile metachro- matic leukodystrophy (often called classical MLD), or the juvenile-onset variants of Niemann-Pick disease, type C, the GM2 gangliosidoses (juvenile Tay-Sachs dis- ease and juvenile Sandhoff disease), or GM1 gangliosidosis. Differentiation from nonmetabolic hereditary ataxias is usually possible on the basis of the presence of other neurologic signs, such as psychomotor retardation or regression, spasticity, or peripheral neuropathy, and evidence of non-neurologic involvement with the disease. The diagnosis in each case is confirmed by demonstrating deficiency of the relevant enzyme activity in plasma, peripheral blood leukocytes, or cultured skin fibroblasts (see Chapter 9).
Coenzyme Q10 (CoQ10)deficiency may also present in young children as a very slowly progressive cerebellar ataxia with muscle weakness and pyramidal tract signs, associated in some cases with seizures and mental retardation. Imaging of the CNS typically shows cerebellar atrophy. Routine biochemical studies levels, are generally normal, though some patients have persistent mild lactic acidosis. Measurements of mitochondrial electron transport chain (ETC) activity often show deficiencies of one or more complexes of the ETC; however, the pattern of the abnormal- ities varies from patient to patient. When ataxia is the dominant cause of dis-
ability, muscle histology and histochemistry is normal. CoQ10 concentrations in muscle are decreased, and treatment with high doses of the cofactor, up to 3000 mg per day, produces dramatic improvements in muscle strength, ataxia, and seizure control.
Choreoathetosis and dystonia Choreoathetosis is a prominent feature of Lesch-Nyhan syndrome, caused by X- linked hypoxanthine phosphoribosyltransferase (HPRT) deficiency. Many, though not all, patients with the disease show psychomotor retardation. Self-mutilation, another feature of the condition in many affected boys, may not be present. The diagnosis is suggested by finding increased uric acid levels in blood and urine, and it is confirmed by specific enzyme assay on leukocytes or fibroblasts. Choreoathetosis and dystonia are characteristic of glutaric aciduria, type I. This disorder, caused by deficiency of glutaryl-CoA dehydrogenase, is characterized by 70 A Clinical Guide to Inherited Metabolic Diseases
Figure 2.14 MRI scans of the brain of an infant with glutaric aciduria, type I. Panel a, Axial T1-weighted MRI scan [TR550/TE11] of a 10-month-old infant showing marked prominence of the Sylvian fissures with hypoplasia of the temporal lobes and generalized delayed myelination. Panel b, Coronal fast spin echo T2-weighted MRI scan [TR4000/TE65] through the enlarged Sylvian fissures showing increased signal intensity of the heads of the caudate nuclei and putamen bilaterally.
the acute onset in early infancy of intermittent episodes of encephalopathy with ketoacidosis, hypotonia, seizures, posturing (arching, grimacing, tongue-thrusting, rigidity), and evidence of hepatocellular dysfunction. Recovery is usually incom- plete, with the extrapyramidal movement disorder tending to persist. The MRI of the brain typically shows cerebral atrophy particularly prominent in the temporal lobes, producing changes that are virtually diagnostic of the disease (Figure 2.14). The urine usually contains large amounts of glutaric acid and smaller amounts of 3- hydroxyglutaric acid, though sometimes the organic acid pattern is not abnormal; the presence of 3-hydroxyglutaric acid is more specific and is particularly important in establishing the diagnosis. Dystonia is a prominent feature of disorders of neurotransmitter metabolism (Table 2.10). In many cases, the disturbance is intermittent and may be asso- ciated with choreoathetosis or parkinsonism. Dopa-responsive dystonia (Segawa syndrome) is characterized by the onset in middle childhood of dystonia, spasticity, and parkinsonian rigidity and tremor, that grows progressively more pronounced 71 Neurologic syndrome
as the day progresses and the patient becomes tired. The response to treatment with L-dopaisdramaticandvirtuallydiagnostic.Theconditionismostoftentransmitted as an autosomal dominant disorder with decreased penetrance and marked variabil- ity in clinical expression of the disease, even among members of the same family. It is caused by mutations in GCH1 resulting in GTP cyclohydrolase deficiency. It is associated with typical abnormalities of CSF neurotransmitter metabolites (see Table 9.7). The diagnosis is confirmed by demonstrating deficiency of the enzyme in cultured skin fibroblasts or phytohemagglutinin-stimulated peripheral blood lymphocytes, or by specific mutation analysis. An autosomal recessive form of the same syndrome is caused by deficiency of tyrosine hydroxylase. The onset of symptoms is earlier, and the course is more rapidly progressive in this variant of Segawa syndrome. It is often associated with intellectual impairment. Nevertheless, the response to treatment with L-dopa is just as dramatic as in patients with the autosomal dominant variant of the disease. Involuntary choreiform movements, progressive hypotonia, developmental delay, dystonia, and intractable seizures presenting in early infancy are prominent features of cerebral creatine deficiency caused by deficiency of guanidinoacetate methyltransferase (GAMT), an autosomal recessive disorder of creatine biosyn- thesis. The plasma and urinary creatinine concentrations may be abnormally low. However, confirmation of the diagnosis requires the demonstration of increased concentrations of guanidinoacetate in plasma, urine, or CSF. MRS of the brain typically shows marked reduction in the size of the creatine peak. Treatment with creatine monohydrate, together with dietary arginine restriction and ornithine supplementation, is reported to produce a significant improvement in seizure control.
Parkinsonism Parkinsonism, dystonia, and cerebellar dysfunction are prominent symptoms in many patients with Wilson disease. Most patients with the disease come to medical attention in later childhood with evidence of severe hepatocellular dysfunction (see Chapter 4). However, many present somewhat later, in adolescence or early adult- hood, with neuropsychiatric problems, usually dominated by extrapyramidal or cerebellar dysfunction, or psychiatric disturbances. Cerebellar ataxia is often asso- ciated with tremors, titubation, dysmetria, and scanning speech. In other patients, other signs of extrapyramidal disease dominate, with dystonia, cog-wheel rigidity, facial grimacing, drooling, dysphagia, and stereotypic gestures. In spite of pro- found disturbances of motor function, intelligence usually remains normal. On the other hand, psychiatric problems are often prominent. They may even be the first indication of disease (see Table 2.18). 72 A Clinical Guide to Inherited Metabolic Diseases
Myopathy
Inherited metabolic disorders presenting as myopathy are commonly the result of defects in energy metabolism. These can be divided into five categories on the basis of the clinical characteristics of the muscle disease and associated findings: r Acute intermittent muscle weakness r Progressive muscle weakness. r Exercise intolerance with cramps and myoglobinuria (myophosphorylase defi- ciency phenotype). r Exercise intolerance with cramps and myoglobinuria (CPT II deficiency pheno- type). r Myopathy as a manifestation of multisystem disease (mitochondrial myopathies).
Acute intermittent muscle weakness Episodes of acute onset of marked muscle weakness, hypotonia and unsteadiness of gait lasting from a few hours to a few days is a feature of hyperkalemic and hypokalemic periodic paralysis. These present a particularly challenging diagnostic problem. Onset is generally in later childhood, with episodes of flaccid muscle weak- ness associated with hyperkalemia or hypokalemia, lasting some hours. Attacks typically occur during rest following intensive exercise. In patients with hyper- kalemic periodic paralysis (HyperPP), attacks may also be precipitated by ingestion of potassium. Between attacks, patients with HyperPP often exhibit myotonia of the facial muscles and extremities. The disease is caused by mutations in the sodium channel gene, SCN4A,inskeletal muscle. Paramyotonia congenita (PMC), which is also caused by SCN4A mutations, is characterized by attacks of muscle stiffening (myotonia) precipitated by exercise or exposure to cold. The stiffness typically evolves into flaccid muscle weakness lasting for some hours before resolving spontaneously. Between attacks, patients show myotonia of the muscles of the face and throat, with delayed eye-opening and percussion myotonia of the tongue. The attacks of weakness in hypokalemic periodic paralysis (HypoPP) are associ- ated with low plasma potassium concentrations. They are typically precipitated by ingestion of a high-carbohydrate meal. Myotonia is not a feature of this disorder, and patients apparently do not develop progressive myopathy later in life. The con- dition is caused by mutations in the calcium channel gene, CACNL1A3,inskeletal muscle. What is remarkable about patients with these disorders is the absence of any evi- dence of acute encephalopathy, systemic illness, or underlying neurological deficits or psychomotor retardation. In some, the diagnosis is suggested by the presence of 73 Neurologic syndrome
typicalelectrolytedisturbances(i.e.,hypokalemiaorhyperkalemia)duringepisodes of weakness. Electrolyte analyses done when the patient is well are almost always normal. Most of these disorders are transmitted as autosomal dominant condi- tions, and the family history may reveal that one or other of the parents will have had similar transient episodes of marked weakness during later childhood, or has developed a slowly progressive myopathy. A predominance of affected males is a characteristic of HypoPP.
Progressive muscle weakness One of the most striking examples of inherited metabolic diseases presenting with progressive myopathy is Pompe disease (GSD II), caused by deficiency of the lysoso- mal enzyme, -glucosidase (acid maltase). It is characterized by the onset, at three to five months, of rapidly progressive weakness and hypotonia. Affected infants are remarkable for the marked paucity of spontaneous movement and their frog-leg posture, but who have normal social interaction. The face is myopathic, and the tongue is characteristically enlarged; however, extraocular movements are spared. Despite marked muscle weakness and hypotonia, muscle bulk is initially not sig- nificantly decreased, and the muscles have a peculiar woody texture on palpation. Deep tendon reflexes, which may initially be preserved, are soon lost. The liver is not significantly enlarged unless the infant is in heart failure. Cardiac muscle involvement is prominent and severe (see Chapter 5). The course of the disease is relentlessly progressive, culminating in death within a few months. The excess glycogen in the muscles of infants with Pompe disease accumulates in lysosomes, and lysosomal glycogen contributes next to nothing to meeting the energy needs of the tissue. Why the muscle weakness in the disease is so severe is not understood. Treatment by enzyme replacement therapy is currently in clinical trials and offers some hope for the families of infants with this otherwise uniformly lethal disease. In late-onset variants of acid maltase deficiency, the onset of the myopathy is more insidious and the progression more gradual. Muscle biopsy typically shows the presence of large accumulations of intra-lysosomal glycogen. Cardiomyopa- thy is much less prominent in late-onset disease; death is invariably the result of respiratory failure caused by respiratory muscle weakness. Progressive skeletal myopathy, sometimes involving the heart, may be a major problem in patients with glycogen storage disease, type III (GSD III). This dis- ease is caused by deficiency of glycogen debrancher enzyme, usually in liver and muscle, but sometimes only in liver. While the consequences of liver involvement usually improve with age (see Chapter 4), the myopathy gradually becomes worse, often only becoming clinically significant after age 20 or 30 years. The creatine phosphokinase (CPK) is often, though not always, elevated in patients with muscle 74 A Clinical Guide to Inherited Metabolic Diseases
involvement. The mechanism of the myopathy in GSD III is uncertain. Some feel that it is the result of local glycogen accumulation; others think, on the basis of apparent improvement using high protein dietary treatment, that increased muscle protein breakdown to fuel gluconeogenesis is responsible. Muscle weakness and exercise-induced myoglobinuria, associated with seizures, ataxia, pyramidal tract signs, and developmental retardation, are particularly
prominent in some cases of CoQ10 deficiency (Table 2.10). Muscle biopsies in patients with this variant of the disorder show the presence of ragged-red fibres, and measurement of mitochondrial electron transport chain (ETC) activity in muscle
or cultured skin fibroblasts typically shows deficiencies of CoQ10-dependent com- plexes I+II and I+III. The diagnosis is confirmed by showing marked deficiency of
CoQ10 in muscle or cultured skin fibroblasts. Treatment with large doses of CoQ10 results in significant improvement in muscle power in most and the ataxia in some
patients with CoQ10 deficiency.
Myoglobinuria (myophosphorylase deficiency phenotype) The clinical course of myophosphorylase deficiency (McArdle disease or GSD V) is typical of a number of inherited defects of glycolysis presenting as exercise intol- erance. It is characterized by the onset in early adulthood of severe muscle cramps shortly after the initiation of intense exercise; mild, sustained exercise, such as level walking, is well-tolerated. Typically, if the patient rests briefly, moderate levels of activity can be resumed without discomfort. This is the so-called ‘second-wind’ phenomenon. Presumably as the muscle switches to fatty acid oxidation in order to meet its energy needs, the requirement for glucose is decreased, and the cramps disappear. Episodes of cramping are often followed within hours by the develop- ment of wine-colored pigmentation of the urine (myoglobinuria) as a result of rhabdomyolysis. CPK levels are typically markedly elevated and rise further dur- ing exercise. Rhabdomyolysis and resulting myoglobinuria occur in all myopathies occurring as a result of defects in skeletal muscle energy metabolism. Rarely, it is severe enough to cause acute renal failure. The normal accumulation of lactic acid in the course of an ischemic forearm exercise test does not occur, and the normal increase in plasma ammonium is exag- gerated. The test (Table 2.11)involves the measurement of lactate and ammonium in blood collected from the antecubital vein before and after a defined period of vigorous exercise during which the circulation to the forearm is temporarily inter- rupted by application of a partially inflated blood pressure cuff. In many patients, discomfort associated with the task forces interruption of the test before completion of the two minutes of vigorous exercise. In that case, blood samples for measurement of lactate and ammonium should continue to be collected as scheduled. 75 Neurologic syndrome
Table 2.11 Protocol for the ischemic forearm exercise test
1. An intravenous is established in one arm with an ample needle in the antecubital vein kept open with a slow infusion of 0.9% NaCl. 2. The patient is given a rolled-up, partially-inflated, blood pressure cuff attached to a sphygmomanometer to squeeze. 3. A second cuff is applied to the arm, above the elbow, but it is not inflated. 4. Blood is taken for analysis of lactate, ammonium, and CPK (baseline). 5. The cuff on the arm is inflated to 120–140 mm of mercury, and the patient is instructed to squeeze the cuff in her hand rapidly (30–60 times per minute), trying as hard as possible to produce a pressure ≥100 mm of mercury. After 2 minutes, the cuff on the arm is deflated and the patient is instructed to relax. 6. Blood samples are obtained from the intravenous line for measurement of lactate, ammonium, and CPK at 2, 5, 10, and 15 minutes after termination of the 2 minutes of ischemic exercise and deflation of the cuff on the arm.
Abbreviations: CPK, creatine phosphokinase.
Muscle phosphofructokinase (PFK) deficiency (GSD VI) shares many features in common with myophosphorylase deficiency, including severe muscle cramp- ing during short-term exercise, a ‘second-wind’ phenomenon, abnormal ischemic forearm exercise test, and myoglobinuria. However, onset in childhood is more common, the attacks of muscle cramps are generally more severe, and they are aggravated by ingestion of high-carbohydrate meals. Like patients with other inborn errors of glycolysis, patients with PFK deficiency generally show evidence of a compensated hemolytic anemia, and it may be associated with marked hyper- uricemia. The disorder is more prevalent among Ashkenazi Jews and Japanese than in people of other ethnic groups. Exercise intolerance of the ‘myophosphorylase deficiency phenotype’ occurs in patients with other glycolytic defects (Table 2.12), but these are rare. Myoadenylate deaminase deficiency is often clinically indistinguishable from myophosphorylase deficiency. However, the average age of onset is somewhat later, and the attacks of exercise-induced cramping tend to be less severe. CPK levels are increased in half the patients, and the electromyogram (EMG) is often normal. The ischemic forearm exercise test produces a normal increase in plasma lactate, but the normal increase in plasma ammonium does not occur. The diagnosis is confirmed by enzyme analysis, specific histochemical staining of the muscle, or mutation analysis. In about half the patients with adenylate deaminase deficiency, the symptoms are due to secondary or acquired enzyme deficiency associated with other chronic neuromuscular problems or with collagen vascular disease. Table 2.12 Inherited metabolic diseases presenting as muscle cramping or myoglobinuria
Disease Clinical features Diagnosis
Myophosphorylase deficiency phenotype Muscle phosphorylase deficiency Muscle cramps during exercise, ‘second-wind’ phenomenon, normal Deficiency of phosphorylase in muscle (McArdle disease) pre-test lactate and no increase on ischemic forearm exercise test, elevated CPK, myoglobinuria PFK deficiency Muscle cramps during exercise, myoglobinuria, hyperuricemia (and Marked deficiency of PFK activity in muscle; gout), excessive increase of ammonium on ischemic forearm exercise half normal activities in erythrocytes test, compensated hemolytic anemia, elevated CPK, more common in Ashkenazi Jews and Japanese PGK deficiency X-linked recessive, chronic hemolytic anemia, mental retardation, Deficiency of PGK in erythrocytes psychiatric problems PGAM deficiency May be clinically indistinguishable from PFK deficiency Deficiency of PGAM in muscle LDH deficiency May be clinically indistinguishable from PFK deficiency. No lactic Deficiency of LDH-M subunit in erythrocytes acidosis, but marked hyperpyruvic acidemia during ischemic forearm exercise test
CPT II deficiency phenotype CPT II deficiency Post-exercise cramps or myalgia, cold-induced muscle cramps and Deficiency of CPT II in fibroblasts stiffness, increased CPK during fasting, normal lactate and ammonium responses but increased CPK on ischemic forearm exercise test, myoglobinuria Myoadenylate deaminase deficiency Post-exercise muscle cramps or myalgia, normal lactate response, but no Deficiency of adenylate kinase on increase in ammonium on ischemic forearm exercise test, elevated histochemical or biochemical analysis of CPK in about 50% skeletal muscle LCAD deficiency Similar to CPT II deficiency, episodes of Reye-like encephalopathy, Deficiency of LCAD in fibroblasts decreased plasma carnitine SCHAD deficiency Extremely rare, chronic muscle weakness with episodic acute Deficiency of SCHAD in muscle. Enzyme deterioration and myoglobinuria, prominent myocardial involvement activity in fibroblasts is normal.
Abbreviations: CPT II, carnitine palmitoyltransferase II; LCAD, long-chain acyl-CoA dehydrogenase; SCHAD, short-chain 3-hydroxyacyl-CoA dehydrogenase; LDH, lactate dehydrogenase; PFK, phosphofructokinase; PGK, phosphoglycerate kinase; PGAM, phosphoglycerate mutase; CPK, creatine phosphokinase. 77 Neurologic syndrome
Table 2.13 Differences between myophosphorylase deficiency and CPT II deficiency phenotypes
Phenotype
Myophosphorylase CPT II
Short-term intense exercise Not tolerated Well tolerated Prolonged mild-moderate exercise Well tolerated Not tolerated Second wind phenomenon Present Absent Effect of fasting Beneficial Detrimental High carbohydrate-low fat diet No benefit∗ Beneficial
Note: ∗Although high carbohydrate dietary treatment is not beneficial in patients with myophos- phorylase deficiency, a high-protein diet appears to be beneficial in some, and ingestion of glucose immediately before exercising often enhances exercise tolerance. Abbreviations: CPT II, carnitine palmitoyltransferase II. Source: Adapted from Di Mauro, Bresolin & Papadimitriou (1984).
Myoglobinuria (CPT II deficiency phenotype) In patients with myopathy resulting from defects in fatty acid oxidation, the muscle cramps and tenderness characteristically develop after periods of exercise, when the patient is actually at rest, and the muscle is drawing heavily on fatty acid oxidation to meet its energy requirements. The main difference between the myophosphorylase deficiency and CPT II defi- ciency phenotypes are shown in Table 2.13.Most patients with CPT II (carni- tine palmitoyltransferase II) deficiency present as young adults with a history of episodic muscle stiffness, pain, tenderness, weakness, and myoglobinuria precip- itated by prolonged exercise, exposure to cold, fasting, or intercurrent infection. Patients do not experience a ‘second-wind’ phenomenon. Between attacks, they may be completely asymptomatic, though some experience residual muscle weak- ness and fatiguability. The CPK is elevated during attacks, but it is generally normal at other times. Muscle biopsy typically shows lipid accumulation in many, though not all, affected individuals. The myoglobinuria is severe enough in many patients to precipitate renal failure. The normal ketotic response to fasting (see Chapter 4) is blunted, though acute metabolic decompensation in older patients is rare. The diagnosis is confirmed by demonstrating deficiency of CPT II activity in fibroblasts.
Myopathy as a manifestation of multisystem disease (mitochondrial myopathies) Progressive myopathy is often the principal manifestation of the multisystem involvement of diseases caused by defects in the mitochondrial ETC, though other systems are invariably involved. The extent and degree of involvement 78 A Clinical Guide to Inherited Metabolic Diseases
of various tissues and organs inpatients with different mitochondrial ETC defects varies enormously, not only between unrelated patients, but also between patients within the same family. In some disorders, such as Leigh subacute necrotizing encephalomyelopathy and Alpers disease (see section on ‘Chronic encephalopathy’), the myopathy is clinically mild compared with the effects of the disease on the CNS. In others, hepatocellular dysfunction or other gastrointestinal disturbances, such as pseudo-obstruction, dominate the presentation. However, in others, muscle weakness may be the presenting symptom and the multisystem nature of the condition is only appreciated after careful examination and labora- tory studies. In most, the course of the disease is relentlessly progressive; in some, it is marked by episodes of acute deterioration superimposed on a background of chronic deterioration; and in a few, mostly young infants, spontaneous recovery occurs over a period of a few months. Most are associated with persistent lactic acidosis, though lactate levels are generally not >10 mmol/L except during acute metabolic decompensation. The mode of inheritance may be autosomal recessive, autosomal dominant, X-linked, or mitochondrial (matrilineal). Among patients with disease due to mitochondrial mutations, a large proportion of them is de novo mutations, rather than inherited. However, the clinical symptomatology associated with mitochondrial mutations is often highly variable owing to the phenomenon of heteroplasmy (see Chapter 1), and other family members should be studied in detail before the mutation in a particular patient is concluded to be new. Charac- teristics that suggest the myopathy is due to a mitochondrial ETC defect are shown in Table 2.14. In spite of the enormous variability between patients with mitochondrial ETC defects, many patients exhibit patterns of clinical findings that have made it possible to identify some relatively distinct syndromes. Many of these are attributable to mtDNA mutations (Table 2.15). However, some conditions characterized by mtDNA abnormalities, and others in which mitochondrial ETC activity is directly involved, are caused by nuclear mutations (Table 2.16). Definitiveinvestigationofthisgroupofdisordersrequiresmusclebiopsywithhis- tochemical studies, electron microscopy, and biochemical studies on mitochondrial electron transport in mitochondria isolated fresh from the tissue (see Chapter 9). The presence of ragged-red fibers in skeletal muscle biopsies of the tissue stained by the modified Gomori trichrome stain (Figure 2.15)isareflection of the prolifer- ation and subsarcolemmal aggregation of mitochondria, which is characteristic of many of the mitochondrial myopathies. Electron microscopic examination often confirms the presence of abnormal square or rectangular paracrystalline inclusions between the inner and outer mitochondrial membranes, or globular inclusions in the matrix. The tissue often contains excess glycogen and fat, sometimes giving the 79 Neurologic syndrome
Table 2.14 Some common clinical features of conditions caused by mitochondrial mutations
Present in most mitochondrial conditions Persistent lactic acidosis Myopathy (weakness, hypotonia) Failure to thrive; short stature Psychomotor retardation; dementia Seizures
Present in many mitochondrial conditions Ophthalmoplegia or other oculomotor abnormalities Retinal pigmentary degeneration Cardiomyopathy Cerebellar ataxia (progressive or intermittent) Sensorineural hearing loss Cardiac arrhythmias Diabetes mellitus Stroke (in children) Renal tubular dysfunction Respiratory abnormalities (periodic apnea and tachypnea)
appearance of a ‘lipid myopathy’,such as is characteristic of the changes in patients with fatty acid oxidation defects. The identification of specific mitochondrial mutations is a growing part of the investigation of mitochondrial disorders, including the mitochondrial myopathies. However, this is not yet routinely available except in a handful of centers doing basic research in the area.
Autonomic dysfunction
In a handful of inherited metabolic diseases, what appear to be abnormalities of autonomic function are particularly prominent (Table 2.17). In some, such as Fabry disease, the autonomic dysfunction may be the first indication of disease; in others, attempts to control problems, such as intractable diarrhea, are the most challenging aspects of the management of the diseases.
Psychiatric problems
Some inherited metabolic disorders in which chronic progressive encephalopathy is prominent are characterized by severe behavior problems. For example, boys with Table 2.15 Main clinical features of some relatively common mitochondrial syndromes resulting from mtDNA mutations
KSS MERRF MELAS NARP LHON
Ophthalmoplegia ++++ 00 0 0 Retinal degeneration ++++ 00 ++++ ++++ Cerebellar dysfunction +++ ++++ 0 +++ ± Psychomotor regression ++ ++ +++ + 0 Myoclonus 0 ++++ 000 Seizures + +++ ++++ ++ 0 Sensorineural deafness +++ ++ + 00 Cortical blindness ± hemiparesis 00++++ 00 (stroke) Renal tubular dysfunction 0 0 ++ 00 Cardiomyopathy ++ + ± ± 0 Cardiac conduction defects ++++ 00 0 + Short stature +++ ++ ++++ 00 Diabetes mellitus 0 0 +++0 Lactic acidosis ++ ++ +++ ± 0 Common mutations Large rearrangements TK∗MERRF8344 TL1∗MELAS3243 ATP6∗NARP8993 ND4∗LHON11778 Positive family history ± +++ ++ +++ +++
Abbreviations: KSS, Kearns-Sayre syndrome; MERRF, myoclonic epilepsy and ragged-red fiber disease; MELAS, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; NARP, neurogenic weakness, ataxia, and retinitis pigmentosa; LHON, Leber’s hereditary optic neuropathy; +,present; ±, variably present; 0, not present. The abbreviations for the common mtDNAmutations indicate the mitochondrial gene involved, the syndrome associated with the mutation, and the nucleotide number where the substitution has occurred. TK and TL1 refer to tRNALys and tRNALeu(UUR),respectively. Table 2.16 Some mitochondrial syndromes caused by nDNA mutations
Mode of Disease Main clinical features Molecular phenotype inheritance Genes involved
Leigh Relapsing acute encephalopathy, progressive Various mitochondrial ETC AR Various cerebral neurodegeneration, lactic acidosis, defects: Complex I, Complex episodic tachypnea, hypotonia, seizures, II, Complex III, Complex IV ±cardiomyopathy, ±hepatocellular (COX) dysfunction, ±renal tubular dysfunction CPEO/KSS Ophthalmoplegia, muscle weakness, Large mtDNA deletions AD or AR sensorineural deafness, cardiomyopathy, cardiac conduction defects, peripheral neuropathy, ataxia Pearson Onset in infancy of failure to thrive, chronic Multiple mtDNA deletions marrow-pancreas diarrhea, lactic acidosis, siderblastic anemia, syndrome hypoparathyroidism; often evolves into KSS MNGIE Ophthalmoplegia, progressive leukodystrophy, Thymidine phosphorylase AR dementia, myopathy, peripheral neuropathy, deficiency; multiple mtDNA diarrhea, intestinal pseudo-obstruction, deletions, depletion, or both malabsorption Wolfram (DIDMOAD)1 Diabetes insipidus, diabetes mellitus, optic Endoglycosidase H-sensitive AR WFS1 atrophy, sensorineural deafness, psychiatric membrane glycoprotein disturbances (see Table 2.17) deficiency; multiple mtDNA deletions in some (cont.) Table 2.16 (cont.)
Mode of Disease Main clinical features Molecular phenotype inheritance Genes involved
Hepatocerebral MDS Early-onset encephalopathy, oscillating eye Deoxyguanosine kinse deficiency; AR DGUOK movements, severe failure to thrive, mtDNA depletion in liver hepatocellular failure, markedly elevated plasma -fetoprotein, lactic acidosis, death in infancy Myopathic MDS Onset in early childhood of progressive skeletal Thymidine kinase deficiency; AR TK2 myopathy, with death in first decade of life mtDNA depletion in muscle Friedreich ataxoa Progressive gait ataxia, pes cavus, Deficiency of frataxin AR FRDA kyphoscoliosis, sensory neuropathy, dysarthria, nystagmus, sensorineural deafness, absent deep tendon reflexes, extensor plantar reflexes, cardiomyopathy, diabetes mellitus in some ARHSP Progressive spastic paraplegia, sensory Primary paraplegin deficiency AR SPG7 neuropathy, ragged-red fibres, with mental with secondary deficiencies of retardation, ataxia, retinitis pigmentosa, Complex I, II/III, and IV deafness, and ichthyosis in some
Abbreviations: CPEO, chronic progressive external ophthalmoplegia; KSS, Kearns-Sayre syndrome; MNGIE, mitochondrial neurogastrointestinal encephalomyopathy and encephalopathy; ETC, electron transport chain; ARHSP, autosomal recessive hereditary spastic paraplegia; AR, autosomal recessive; AD, autosomal dominant; MDS, mitochondrial DNA depletion syndrome; 1 In most cases of Wolfram syndrome, the mtDNA is intact. 83 Neurologic syndrome
Figure 2.15 Photomicrograph of skeletal muscle stained by the modified Gomori trichrome method showing ragged-red fibers. The bar represents 50 m. (Courtesy of Dr. Venita Jay, Toronto, Canada.)
Hunter disease (MPS II) and children with Sanfilippo disease (MPS III) exhibit par- ticularly severe hyperactivity, impulsiveness, short attention span, poor tolerance of frustration, aggressiveness, and sleeplessness. In Sanfilippo disease, the extraor- dinarily disruptive behavior may be what brings the patient to medical attention. The implacable irritability of infants with Krabbe globoid cell leukodystrophy is a prominent feature of the disease. Infants with hepatorenal tyrosinemia commonly exhibit acute episodes of extreme irritability, often accompanied by acute onset of reversible peripheral neuropathy, all attributable to the secondary porphyria which is a feature of the disease. In many other inherited metabolic diseases, particularly late-onset disorders, the first indication of the onset of disease may be behavioral or personality changes (Table 2.18). The earliest signs of the disease in boys affected with X-linked adrenoleukodystrophy are often social withdrawal, irritability, obsessional behav- ior, and inflexibility. Patients with adult-onsetmetachromatic leukodystrophy may present with subtle evidence of chronic organic brain syndrome, such as anxiety, depression, emotional lability, social withdrawal, disorganized thinking, and poor memory. In late-onset variants of GM2 gangliosidosis, the patient may present with a frank psychosis characterized by severe agitation, obsessional paranoia, Table 2.17 Inherited metabolic disease in which autonomic dysfunction is prominent
Disease Clinical features Diagnosis
Dopamine -hydroxylase Adolescent- or adult-onset severe orthostatic hypotension. CSF neurotransmitter analysis: ↑HVA, normal 5HIAA, deficiency ↓MHPG, normal 3OMD, normal BH4, normal BH2, normal Neopterin Neurovisceral porphyrias Abdominal pain, vomiting, autonomic instability, hypertension, See Chapter 9 constipation, diarrhea, tachycardia, bladder dysfunction Fabry disease Severe, intermittent, paresthesias and acroparesthesias of hands Deficiency of lysosomal -galactosidase A in plasma or and feet, hypohidrosis, chronic diarrhea, postural leukocytes (males); increased urinary CTH (males hypotension or females); demonstration of GLA mutations (males or females) MPS I, MPS II, MPS III Chronic, frequent, loose stools, along with dysmorphic facies, Deficiency of appropriate lysosomal enzyme activity in dysostosis multiplex, hepatosplenomegaly, and mental leukocytes or cultured skin fibroblasts retardation (see Chapter 6) Occipital horn syndrome Postural hypotension, marked hypotonia, chronic diarrhea, Decreased concentrations of copper and ceruloplasmin along with cutaneous hyperkeratosis, skeletal abnormalities in plasma (see Chapter 6) MNGIE Early adult-onset recurrent diarrhea, intestinal Deficiency of thymidine phosphorylase in leukocytes pseudo-obstruction, cachexia, along with ophthalmoparesis, peripheral neuropathy, leukoencephalopathy
Abbreviations:CSF,cerebrospinalfluid,HVA,homovanillicacid;5HIAA,5-hydroxyindoleaceticacid;MHPG,3-methoxy-4-hydroxyphenylglycol;3OMD, 3-O-methyldopa; BH4, tetrahydrobiopterin; BH2, 7,8-dihydrobiopterin; MPS, mucopolysaccharidosis; MNGIE, mitochondrial neurogastrointestinal encephalomyopathy; CTH, ceramide trihexoside. 85 Neurologic syndrome
Table 2.18 Some inherited metabolic diseases characterized by psychiatric or severe behavioral abnormalities
Disease Psychiatric/behavioral abnormality
Children Sanfilippo disease (MPS III) Extreme hyperactivity, impulsivity, poor tolerance of frustration, aggressiveness, sleeplessness Hunter disease (MPS II) Extreme hyperactivity, impulsivity, poor tolerance of frustration, aggressiveness, sleeplessness X-linked ALD Social withdrawal, irritability, obsessional behavior, and rigidity Lesch-Nyhan syndrome Severe self-mutilatory behavior
Adolescents and adults Late-onset MLD Anxiety, depression, emotional lability, social withdrawal, disorganized thinking, poor memory, schizophrenia Late-onset GM2 Acute psychosis with severe agitation, obsessional paranoia, gangliosidosis hallucinations, stereotypic motor automatisms PorphyriaChronic anxiety and depression, and marked restlessness, insomnia, depression, paranoia, and sometimes, hallucinations (during acute crises) Wilson disease Anxiety, depression, schizophrenia, manic-depressive psychosis, antisocial behavior Wolfram syndrome Depression, paranoia, auditory or visual hallucinations, (DIDMOAD) violent behavior, dementia (in some), suicide Cerebrotendinous Delusions, hallucinations, catatonia, dependency, irritability, xanthomatosis agitation, and aggression UCED Periodic acute agitation, anxiety, hallucinations, paranoia Homocystinuria (MTHF Acute ‘schizophrenia’ reductase deficiency) Homocystinuria Acute onset of auditory and visual hallucinations, paranoia (cystathionine -synthase deficiency) Adult-onset NCL (Kuf disease) Thought disorder, flat affect, paranoia with hallucinations, delusions, inappropriate behavior
Abbreviations: ALD, adrenoleukodystrophy; MLD, metachromatic leukodystrophy; UCED, urea cycle enzyme defects; DIDMOAD, diabetes insipidus-diabetes mellitus-optic atrophy-deafness; MTHF, methylenetetrahydrofolate; NCL, neuronal ceroid lipofuscinosis. hallucinations, and stereotypic motor automatisms. Personality changes are a com- mon feature of Wilson disease, but usually only after the development of other neurologic manifestations of the disease. Three of our older patients with HHH syn- drome have experienced periodic episodes of acute hallucinatory states lasting up to afew hours. The episodes generally occurred during periods of hyperammonemia, 86 A Clinical Guide to Inherited Metabolic Diseases
and the frequency of attacks decreased with improved metabolic control; however, their occurrence did not correlate well with the severity of the hyperammonemia. Psychiatric disturbance, including early signs of dementia, are a particularly common presentation for late-onset variants of conditions typically presenting as rapidly progressive psychomotor regression in more common, childhood-onset variants of the same diseases. In many cases, the pyschiatric problems may be the only, or at least the predominant, sign of disease for years. The characterization of the disturbances in this group of patients is still incomplete. However, signs that increase the suspicion of dementia caused by an inherited metabolic disorder include: r onset before age 40 years r apositive family history, especially in a sibling r presence of hard neurological signs, specifically extrapyramidal abnormalities r presence of white-matter abnormalites on MRI r prominence of executive dysfunction on neuropsychological testing Acute porphyria may present as an acute psychiatric emergency in patients with acute intermittent porphyria (AIP), the commonest inherited metabolic disorder of porphyrin biosynthesis everywhere except South Africa, where variegate por- phyria affects 1 of every 250 individuals in the Afrikaans population. Typically, patients present with acute neurovisceral crises, with delirium, psychosis, auto- nomic instability (severe constipation, tachycardia, hypertension), abdominal pain, electrolyte disturbances, muscle weakness, and peripheral neuropathy, often when exposed to certain drugs, such as barbiturates, sulfonamides, or alcohol. However, the neuropsychiatric manifestations of the disease are extraordinarily protean, and the diagnosis of porphyria should be considered in any situation where the psy- chiatric symptoms include evidence of acute organic brain syndrome, resistance to conventional treatment, or association with exposure to known porphyrinogenic drugs. Although severe, acute, neurovisceral crises may cause death, only 20% of individuals who inherit AIP-causing mutations of the porphobilinogen deaminase gene actually develop any symptoms of the disease. Women are much more likely to become symptomatic than men, especially during the luteal phase of the menstrual cycle. The development of symptoms before puberty is unusual. Asummary of the metabolism of porphyrins and related inborn errors of por- phyrin metabolism is presented in Figure 9.2 and Table 9.9.Some generalizations: r the neurovisceral manifestations of porphyria are caused by accumula- tion of water-soluble, porphyrin precursors, porphobilinogen (PBG) and 5- aminolevulinic acid (ALA), not any of the more distal intermediates in heme biosynthesis; r the cutaneous manifestions of the disease are caused by accumulation of metabo- lites distal to PBG or ALA; 87 Neurologic syndrome
r hereditary coproporphyria and variegate porphyria are often associated with neurovisceral symptoms because coprophyrinogen III and protoporphyrino- gen IX inhibit PBG deaminase, causing secondary accumulation of PBG and ALA; r the more distal porphyrin metabolites, such as coproporphyrinogens III and pro- toporphyrinogen IX, are less water-soluble than the more proximal metabolites, such as PBG and ALA, which is why the concentrations of these metabolites are higher in stool than in urine; and r urinary porphyrin metabolites may only be present in excess for a few days, during the acute attack – fecal abnormalities tend to persist for several days, sometimes indefinitely. An approach to the laboratory investigation of porphyria is presented in Chapter 9.
SUGGESTED READING
Aicardi, J. (1993). The inherited leukodystrophies clinical overview. Journal of Inherited Metabolic Diseases, 16, 733–43. Argov, Z. & Arnold, D. L. (2000). MR spectroscopy and imaging in metabolic myopathies. Neurologic Clinics, 18, 35–52. Assmann, B., Surtees, R. & Hoffmann, G. F. (2003). Approach to the diagnosis of neurotrans- mitter diseases exemplified by the differential diagnosis of childhood-onset dystonia. Annals of Neurology, 54 (suppl 6), S18–S24. Batshaw, M. L. (1994). Inborn errors of urea synthesis. Annals of Neurology, 35, 133–41. Chaves-Carballo, E. (1992). Detection of inheritedneurometabolic disorders. A practical clinical approach. Pediatric Clinics of North America, 39, 801–20. Crimlisk, H. L. (1997). The little imitator – porphyria: a neuropsychiatric disorder. Journal of Neurology, Neurosurgery, and Psychiatry, 62, 319–28. De Vivo, D. C. & Johnston, M. V. (eds) (2003). Pediatric neurotransmitter diseases. Annals of Neurology, 54 (Suppl 6), S1–S109. DiMauro, S., Bresolin, N. & Papadimitriou, A. (1984). Fuels for exercise: Clues from disorders of glycogen and lipid metabolism. In Neuromuscular Diseases,ed. G. Serratgrice et al., pp. 45–50. NewYork: Raven Press. DiMauro, S., Bonilla, E. & De Vivo, D. C. (1999). Does the patient have a mitochondrial encephalomyopathy? Journal of Child Neurology, 14 (Suppl 1), S23–35. DiMauro, S. & Schon, E. A. (2003). Mitochondrial respiratory-chain diseases. NewEngland Journal of Medicine, 348, 2656–68. Estrov, Y., Scaglia, F. & Bodamer, O. A. (2000). Psychiatric symptoms of inherited metabolic disease. Journal of Inherited Metabolic Diseases, 23, 2–6. Finsterer, J. (2004). Mitochondriopathies. European Journal of Neurology, 11, 163–86. 88 A Clinical Guide to Inherited Metabolic Diseases
Folstein, M. F., Folstein, S. E. & McHugh, P. R. (1975). “Mini-mental state”. A practical method for grading the cognitive state of patients for the clinician. Journal of Psychiatric Research, 12, 189–98. Goebel, H. H. & Wisniewski, K. E. (2004). Current state of clinical and morphological features in human NCL. Brain Pathology, 14, 61–9. Haltia, M. (2003). The neuronal ceroid-lipofuscinoses. Journal of Neuropathology and Experi- mental Neurology, 62, 1–13. Hyland, K. (1999). Presentation, diagnosis, and treatment of the disorders of monoamine neu- rotransmitter metabolism. Seminars in Perinatology, 23, 194–203. Kahler, S. G. & Fahey, M. C. (2003). Metabolic disorders and mental retardation. American Journal of Medical Genetics, Part C: Seminars in Medical Genetics, 117, 31–41. Kaye,E.M.(2001). Update on genetic disorders affecting white matter. Pediatric Neurology, 24, 11–24. Kelly, P. J., Furie, K. L., Kistler, J. P. et al. (2003). Stroke in young patients with hyperhomocys- teinemia due to cystathionine beta-synthase deficiency. Neurology, 60, 275–9. Kullmann, D. M. (2002).The neuronal channelopathies. Brain, 125, 1177–95. Leonard, J. V. & Schapira, A. H. (2000). Mitochondrial respiratory chain disorders I: mitochon- drial DNA defects. Lancet, 355, 299–304. Leonard, J. V. & Schapira, A. H. (2000). Mitochondrial respiratory chain disorders II: neurode- generative disorders and nuclear gene defects. Lancet, 355, 389–94. Lyon, G., Adams, R. D. & Kolodny, E. H. (1996). Neurology of Hereditary Metabolic Diseases of Children,2nd ed. New York: McGraw-Hill. Parker, C. C. & Evans, O. B. (2003). Metabolic disorders causing childhood ataxia. Seminars in Pediatric Neurology, 10, 193–9. Pavlakis, S. G., Kingsley, P. B. & Bialer, M. G. (2000). Stroke in children: genetic and metabolic issues. Journal of Child Neurology, 15, 308–15. Powers, J. M. & Moser, H. W. (1998). Peroxisomal disorders: genotype, phenotype, major neu- ropathologic lesions, and pathogenesis. Brain Pathology, 8, 101–20. Prasad, A. N. & Prasad, C. (2003). The floppy infant: contribution of genetic and metabolic disorders. Brain & Development, 27, 457–76. Rubio-Gozalbo M E, Dijkman K P, van den Heuvel L P, Sengers R C, Wendel U, Smeitink J A. (2000). Clinical differences in patients with mitochondriocytopathies due to nuclear versus mitochondrial DNA mutations. Human Mutation, 15, 522–32. Schiffmann, R. & van der Knaap, M. S. (2004). The latest on leukodystrophies. CurrentOpinion in Neurology, 17, 187–92. Skladal, D., Sudmeier, C., Konstantopoulou, V. et al. (2003). The clinical spectrum of mitochon- drial disease in 75 pediatric patients. Clinical Pediatrics (Philadelphia), 42, 703–10. Strub, R. L. & Black, F.W.(1999). The mental status examination in neurology,4th ed. Philadelphia: F. A. Davis. Surtees, R. (2000). Inherited ion channel disorders. European Journal of Pediatrics, 159 (Suppl 3), S199–S2003. Tein, I. (1999). Neonatal metabolic myopathies. Seminars in Perinatology, 23, 125–51. 89 Neurologic syndrome
Thunell, S., Harper, P., Brock, A. & Petersen, N. E. (2000). Porphyrins, porphyrin metabolism and porphyrias. II. Diagnosis and monitoring in the acute porphyrias. Scandinavian Journal of Clinical Investigation, 60, 541–59 Tsujino, S., Nonaka, I. & DiMauro, S. (2000). Glycogen storage myopathies. Neurologic Clinics, 18, 125–50. Vedanarayanan, V.V.(2003). Mitochondrial disorders and ataxia. Seminars in Pediatric Neurology, 10, 200–9. 3 3 Metabolic acidosis
Metabolic acidosis is a common presenting or coincident feature of many inherited metabolic diseases. In some cases, the acidosis is persistent, though so mild that the generally recognized clinical signs, such as tachypnea, are absent or so subtle that they are missed. In other cases, the patient presents with an episode of acute, severe, even life-threatening, acidosis, and the underlying persistence of the condition is only recognized after resolution of the acute episode. Diagnostically, the most frustrating presentation is infrequent bouts of recurrent, acute acidosis separated by long intervals of apparent good health during which diagnostic tests show no significant abnormality. This is a particularly challenging situation.
Buffers, ventilation, and the kidney
The hydrogen ion concentration, [H+], of body fluids is maintained within very narrow limits by a combination of buffers, acting immediately, pulmonary venti- lation to restore the capacity of blood buffers, and renal mechanisms to eliminate excess H+. Quantitatively, the most important buffers in blood are the proteins, both the plasma proteins and hemoglobin. Alterations in the concentrations of these pro- teins, particularly hemoglobin, may seriously compromise the capacity of the body to cope with sudden accumulation of acid. The buffering contributed by the equilib- − rium between HCO3 and H2CO3 is important because the capacity of the system is rapidly restored by elimination of H2CO3 through conversion to CO2 and expulsion of the excess CO2 by increased pulmonary ventilation. The buffering properties of the bicarbonate-carbonic acid system are shown by the familiar Henderson-Hasselbach equation:
− [HCO ] pH = pK + log 3 [H2CO3] + CO2(d)
90 91 Metabolic acidosis
pK = aconstant = 6.10 in arterial blood; CO2(d) = concentration of dissolved CO2. In the presence of carbonic anhydrase, H2CO3 is rapidly converted to H2O and CO2.Theconcentration of H2CO3 is, therefore, directly proportional to the con- centration of CO2,which is a function of the partial pressure of CO2, the PaCO2, in blood. The pH and PaCO2 of blood are easily measured, and with that infor- − mation, the [H2CO3 ] can be calculated. The equation is often re-written to show the relationship between its components in terms of the variables that are easily measured: − [HCO ] pH = pK + log 3 (3.1) S × PaCO2
PaCO2 = partial pressure of CO2 in arterial blood; S = aconstant. Without having to recall any specific numbers, one can easily see that an increase in [H+], in the absence of any other change, would cause a decrease in pH. However, + − − association of the H with HCO3 to form H2CO3 causes a decrease in [HCO3 ] and increase in PaCO2,tending to restore the pH. Removal of the excess CO2 by + − increased ventilation permits the association of more H with HCO3 to form more H2CO3, though the total CO2 and, therefore, the total buffer capacity of the system, is decreased in the process. Restoring the buffer capacity of the system requires removal of the excess H+ by some other mechanism. This critical function is carried out by the kidney. − The kidney plays two important roles in acid-base balance: it conserves HCO3 (and sodium), and it secretes H+.Inthe proximal convoluted tubule, 99% of filtered − HCO3 is reabsorbed, along with sodium, amino acids and peptides, glucose, and − phosphate. Loss of HCO3 ,asaresult of damage to the proximal convoluted tubule, decreases the buffering capacity of the bicarbonate-carbonic acid system. In the distal convoluted tubule of the nephron, H+ is secreted by a mechanism involving + + exchange with K and the production and secretion of NH4 and glutamine. Decreased H+–K+ exchange, with increased K+ losses in the urine, is the reason chronic metabolic alkalosis causes potassium depletion. Metabolic acidosis is diagnosed by measurement of blood gases. The typical changes are: r + Decreased arterial blood pH, caused by accumulation of H . r + − Decreased plasma bicarbonate, as excess H is buffered by HCO3 with a shift in − the equilibrium between H2CO and H2CO3. r 3 Decreased PaCO2,owing to compensatory hyperventilation. WhentheaccumulationofexcessH+ isrelativelysmall,respiratorycompensation is usually complete, restoring the blood pHtonormal. However, with increasing H+ accumulation, respiratory compensation becomes insufficient to restore the blood 92 A Clinical Guide to Inherited Metabolic Diseases
pHcompletely to normal. In practical terms, it is rarely possible to decrease the
PaCO2 below 16 mm Hg by increased respiratory effort alone. If respiratory com- pensation is incomplete, from associated pulmonary disease or from respiratory failure, a mixed metabolic respiratory acidosis develops, characterized by increased
PaCO2.Aggressive correction of metabolic acidosis, especially by administration of large amounts of sodium bicarbonate, is often accompanied by the development of respiratory alkalosis as result of persistence of central nervous system (CNS) aci- dosis after correction of the systemic acid-base disturbance. This is rarely a major problem except perhaps in the management of inborn errors of pyruvate oxidation, such as pyruvate carboxylase deficiency, in which CNS production of lactate may be enormous.
Is the metabolic acidosis the result of abnormal losses of bicarbonate or accumulation of acid?
Aglance at the Henderson-Hasselbach equation shows that the drop in pHoccur- ring with metabolic acidosis may occur as a result of either abnormal losses of bicarbonate, or abnormal accumulation of H+,generally in association with some relatively non-volatile organic anion. One way to tell the difference is to calcu- late the concentration of unmeasured anion, the anion gap, which is the differ- + − − ence between the plasma [Na ] and the sum of the plasma [Cl ] and [HCO3 ]. The normal anion gap is 10–15 mEq/L. Albumin is quantitatively the most impor- tant unmeasured anion in plasma. Lactate, acetoacetate, 3-hydroxybutyrate, phos- phate, sulfate, and other minor anions also contribute to the normal anion gap. When metabolic acidosis occurs as a result of bicarbonate losses, either because of renal tubular dysfunction or gastrointestinal losses from diarrhea, the anion gap is − usually normal, in spite of decreased [HCO3 ], owing to an increase in the plasma [Cl−]. Hyperchloremic acidosis is, therefore, one of the hallmarks of metabolic acidosis occurring as a result of abnormal bicarbonate losses.
Metabolic acidosis caused by abnormal bicarbonate losses
A history of diarrhea is usually sufficient to distinguish hyperchloremic metabolic acidosis due to excessive gastrointestinal bicarbonate losses from that arising from renal tubular dysfunction. However, the situation may become confusing if the urine pHisdiscovered to be inappropriately high. The combination of acidosis and hypokalemia, owing to excessive gastrointestinal fluid and electrolyte losses, promotes renal ammonium production and excretion, increasing the urinary pH. By contrast, in patients with inappropriately high urinary pHasaresult of renal tubular acidosis, the urine ammonium concentration is low. Urinary ammonium 93 Metabolic acidosis
Table 3.1 Inherited metabolic diseases associated with renal tubular acidosis (RTA)
Disease Defect
Galactosemia Galactose-1-phosphate uridyltransferase deficiency Hereditary fructose intolerance Fructose-1-phosphate aldolase deficiency Hepatorenal tyrosinemia Fumarylacetoacetase deficiency Cystinosis Defect in cysteine transport out of lysosomes Glycogen storage disease, type I Glucose-6-phosphase deficiency Fanconi-Bickel syndrome Defect in glucose and galactose transport Congenital lactic acidosis Cytochrome c oxidase deficiency Wilson disease Copper transporter defect Vitamin D dependency Cholecalciferol 1 -hydroxylase deficiency Osteopetrosis with RTA Carbonic anhydrase II deficiency Lowe syndrome Phosphatidylinositol-4,5-bisphosphate 5-phosphatase deficiency
concentrations, which are difficult to measure directly, can be estimated by calcu- lating the urine net charge (UNC): [Na+ + K+]–[Cl−]inurine.Anegative UNC is taken as an indication of the presence of ammonium, suggesting the acidosis is the result of abnormal gastrointestinal losses of bicarbonate (and potassium). This method for estimating urinary ammonium concentrations does not apply when the acidosis is the result of accumulation of organic anion. The inappropriately high, though not necessarily alkaline, urinary pHinpatients with proximal renal tubular dysfunction is the result of excessive urinary losses of bicarbonate. In addition to bicarbonate, the reabsorption of amino acids, glucose, phosphate, and urate (renal Fanconi syndrome) is also impaired. The urine may test positive for glucose and reducing substances, and chromatographic analysis shows generalized amino aciduria (see Chapter 9). The plasma phosphate and urate concentrations are also below normal. Renal Fanconi syndrome is a common manifestation of several inherited metabolic diseases (Table 3.1). In some condition, such as cystinosis, Fanconi- Bickel syndrome, and Lowe syndrome, it may be a prominent clinical feature of the disease. In most, the clinical signs of disease are typically dominated by other problems, rather than to the acidosis or renal disease per se.For example, the renal tubular problems in patients with galactosemia or hepatorenal tyrosinemia are usually discovered incidentally; they are rarely the presenting problem. In GSD I, and in hereditary fructose intolerance, the metabolic acidosis caused by accumu- lation of lactic acid is much more prominent than that caused by renal tubular dysfunction. 94 A Clinical Guide to Inherited Metabolic Diseases
Chronic metabolic acidosis, whether it is attributable to bicarbonate losses or to accumulation of anion, is commonly associated with failure to thrive. Patients are often reported to be ‘sickly’ and to have exaggerated difficulties with apparently trivial intercurrent illnesses. Developmental delay is common, but rarely severe, and it is often noted to affect gross motor skills more than speech or socialization. When it is severe and persistent, as it is in infantile cystinosis, metabolic acidosis arising from proximal renal tubular disease is invariably associated with marked growth retardation. Excessive renal tubular loss of phosphate causes rickets.
Metabolic acidosis resulting from accumulation of organic anion
Metabolic acidosis resulting from accumulation of organic anion, caused by inborn errors of organic acid metabolism, is usually persistent. Clinically, it is commonly associated with marked failure to thrive. In addition, persistent, mild metabolic acidosis is often punctuated by intermittentepisodes of acute metabolic decompen- sation. Acute metabolic acidosis causes tachypnea, often without obvious dyspnea. Breathing is rapid and deep, but often it is apparently effortless, and the severity of the respiratory distress may not be appreciated. Secondary hypoglycemia and hyperammonemia, along with accumulationoforganic anion, commonly produce acute encephalopathy with anorexia and vomiting, lethargy, ataxia, and drowsiness progressing to stupor and coma (see Chapter 2). The accumulation of organic anion is often accompanied by a peculiar odor of the sweat or urine. Diagnostically the most important thing to do in patients presenting with metabolic acidosis and an increased anion gap is to identify the unmeasured anion. This is done by a combination of analysis of specific anions, such as lactate, 3-hydroxybutyrate and acetoacetate, and screening procedures, such as analysis of urinary organic acids (Figure 3.1) (see Chapter 9).
Lactic acidosis
Abnormal accumulation of lactic acid is by far the commonest cause of patho- logic metabolic acidosis in children. In the majority of cases, it is caused by tissue hypoxia resulting from inadequate oxygensupply or poor circulation, so-called ‘typeAlactic acidosis’. It occurs in any situation in which the delivery of oxygen to tissues is impaired, such as shock, heart failure, congenital heart disease (especially that producing severe left outlet obstruction), or pulmonary hypertension. Lactic acidosis from hypoxemia may be very severe, with plasma lactate levels in excess of 30 mmol/L, and it is associated with an increase in the lactate to pyruvate ratio (L/P ratio) in plasma. The cause of the lactic acidosis is usually obvious, and the 95 Metabolic acidosis
Figure 3.1 Approach to the investigation of metabolic acidosis.
acidosis is generally reversed within minutes to a few hours by correction of the hypoxic state. The lactic acidosis associated with cardiomyopathy presents a special diagnostic challenge because the cardiomyopathy itself may be due to a primary inherited defect in lactate metabolism (Chapter 5). A clinical classification of lactic acidosis is presented in Table 3.2. Lactate is a ‘dead-end’ metabolite: it is eliminated metabolically by the same route it is formed – through the formation of pyruvate. In addition to H+, the reaction involves two sets of substrates and products: pyruvate/lactate and NADH/NAD+. The conversion is catalyzed by lactate dehydrogenase (LDH), which is ubiquitous and catalyzes the forward and reverse reactions equally well, so that the equilibrium concentration of lactate is directly related to the concentration of pyruvate and the ratio of the concentrations of NADH and NAD+:
+ [NADH] [Lactate] [Pyruvate] × [H ] × [NAD+] It follows that lactate accumulation may occur as a result of pyruvate accumula- tion or NADH accumulation, both tending to push the reaction to the right, or as aresult of H+ accumulation.
Pyruvate accumulation Pyruvate and lactate are the end products of glycolysis, the major source of energy when availability of oxygen is low and in tissues, like erythrocytes, that do not 96 A Clinical Guide to Inherited Metabolic Diseases
Table 3.2 Clinical classification of lactic acidosis
Acquired Inborn errors of metabolism
Hypoxemia Primary Circulatory collapse Defects of pyruvate metabolism Shock PDH deficiency Congestive heart failure Pyruvate carboxylase deficiency Defects of NADH oxidation Severe systemic disease Mitochondrial ETC defects Liver failure Kidney failure Secondary Diabetic ketoacidosis Disorders of gluconeogenesis Acute pancreatitis GSD, type I Acute leukemia HFI PEPCK deficiency Intoxication Fructose-1,6-diphosphatase deficiency Ethanol Fatty acid oxidation defects Methanol Defects of biotin metabolism Ethylene glycol Biotinidase deficiency Oral hypoglycemic drugs Holocarboxylase synthetase deficiency Acetylsalicylic acid Defects of organic acid metabolism Nutritional deficiency HMG-CoA lyase deficiency Thiamine deficiency Propionic acidemia Methylmalonic acidemia Others
Abbreviations: PDH, pyruvate dehydrogenase; GSD, glycogen storage disease; HFI, hereditary fructose intolerance; PEPCK, phosphoenolpyruvate carboxykinase; HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA. Source: Modified from Lehotay & Clarke (1995).
contain mitochondria. Many of the reactions that make up the process of glycolysis are freely reversible and contribute well to gluconeogenesis, the process by which glucose is produced from pyruvate and amino acids (see Chapter 4). Although the sequence of reactions and the regulationofthe rate and direction of metabolic flux are complicated, clinically important aspects of the process can be summarized in afew generalizations and the whole treated as a ‘black box’. The key features of glycolysis are: r It is a cytoplasmic process. r Each molecule of glucose (six carbons), which is uncharged, is converted to two molecules of pyruvic acid (three carbons each), which are negatively charged. r It results in the net production of two molecules of ATP per molecule of glucose. 97 Metabolic acidosis
Figure 3.2 Metabolic sources and fates of pyruvate. The enzymes involved in pyruvate metabolism are: 1, lactate dehydrogenase; 2, pyruvate dehydrogenase complex; 3, pyruvate carboxylase; 4, alanine aminotransferase.
r Overall flux is increased by an intracellular energy deficit and is decreased by signals indicating the concentrations of high energy compounds, like ATP, are adequate. r It produces various intermediates, such as glycerol, required for the synthesis of compounds like triglycerides. Among the inherited metabolic diseases, lactic acidosis due to pyruvate accu- mulation may occur as a result of increased pyruvate production by increased gly- colytic flux. Increased pyruvate production is the mechanism of the lactic acidosis in patients with GSD I, or hereditary fructose intolerance, as a consequence of increased intracellular concentrations of stimulatory phosphorylated intermedi- ates, like fructose-2,6-bisphosphate and fructose-1,6-bisphosphate, respectively. Lactic acidosis also occurs as a consequence of decreased oxidation of pyruvate. Pyruvate, produced from glycolysis, or from the transamination of alanine, is either oxidized to acetyl-CoA, in a reaction catalyzed by the pyruvate dehydrogenase com- plex (PDH), or it is carboxylated to form oxaloacetate, in a reaction catalyzed by the biotin-containing enzyme, pyruvate carboxylase (PC) (Figure 3.2). Whether PDH or PC activity predominates at any particular moment is, as one might expect, determined by the energy needs of the cell. In general, PDH is stimulated by signals indicating an increased need for energy, such as low ATP/ADP ratios; PC is stim- ulated by indications, such as increased acetyl-CoA levels, that the concentration 98 A Clinical Guide to Inherited Metabolic Diseases
of TCA intermediates, particularly oxaloacetate, is too low to support continued operation of the cycle.
PDH deficiency Persistent lactic acidosis is a prominent feature of PDH deficiency. PDH is a huge multicomponent enzyme complex made up of multiple units of four enzymes:
pyruvate decarboxylase (E1,30units), dihydrolipoyl transacetylase (E2,60units), dihydrolipoamide dehydrogenase (E3,6units), and protein X (6 units). Enzyme activity is regulated in part by phosphorylation (inactivation)-dephosphorylation (activation), reactions catalyzed by PDH kinase and PDH phosphatase, respectively.
Most patients with PDH deficiency have mutations of the X-linked E1 subunit of the pyruvate decarboxylase component of the enzyme complex. Nonetheless, males and females are equally represented, except among those patients with the relatively benign form of the disease, which is characterized by intermittent ataxia (see Chapter 2). The clinical course of PDH deficiency is highly variable. The disease may present inthenewbornperiodasseverepersistentlacticacidosis(seeChapter7)terminating in death within a few weeks or months. This variant of the disease is often associated with agenesis of the corpus callosum. Most children with PDH deficiency present later in infancy with a history of psychomotor retardation, hypotonia, failure to thrive, and seizures. The course of the disease is often punctuated by bouts of very severe lactic acidosis, often precipitated by intercurrent infections. Some show subtly dysmorphic facial features. Other patients present with classical Leigh disease (see Chapter 2). Plasma lactate levels in PDH deficiency are persistently elevated, and the acido- sis is generally made worse by ingestion of carbohydrate. However, the L/P ratio is characteristically normal. The plasma alanine level is elevated, a reflection of increased pyruvate concentrations. Urinary organic acid analysis in patients with
E1 defects is unremarkable apart from the presence of excess lactate and some 2-hydroxybutyrate, an organic acid found in the urine of patients with severe lactic acidosis, regardless of the cause. The diagnosis is confirmed by demonstrating PDH deficiency in cultured fibroblasts. Rarely, the PDH deficiency may be the result of
adefectintheE3 component (dihydrolipoamide dehydrogenase) of the enzyme complex. Clinically, affected patients are indistinguishable from patients with E1 defects, though presentation in the newborn period has never been reported. Because both branched-chain 2-ketoacid dehydrogenase and 2-ketoglutarate
dehydrogenase also contain the same E3 subunit as PDH, patients with E3 defects have elevated plasma levels of branched-chain amino acids, though not as high as in maple syrup urine disease (MSUD), and the urinary organic acid anal- ysis shows increased concentrations of 2-ketoglutarate, 2-hydroxyglutarate, and 99 Metabolic acidosis
2-hydroxyisovalerate. A small number of patients with classical Leigh disease have been found to have PDH phosphatase deficiency. Although a few vitamin-responsive variants of PDH deficiency have been reported, treatment of this group of disorders is usually unsatisfactory. However, boys with the benign variant often do better on a high fat, low carbohydrate diet. The lactic acidosis in some patients is relieved to some extent by treatment with the pyruvate analogue, dichloroacetate, which increases PDH activity by inhibiting PDH kinase.
PC deficiency Persistent lactic acidosis is also a prominent feature of PC deficiency. PC is a biotin- dependent enzyme that catalyzes the carboxylation of pyruvate to form oxaloac- etate. It is dependent for activity on the presence of acetyl-CoA. In addition to its role in fueling the TCA cycle, PC catalyzes the first, and most important, reaction in gluconeogenesis (see Chapter 4). PC deficiency is very rare. The most common variant of the disorder (type A) commonly presents in the first few months of life with a history of psychomo- torretardation and signs of intermittent acute metabolic acidosis. Despite the central role PC plays in gluconeogenesis, hypoglycemia is not as a rule a promi- nent feature of the disease. The majority of patients in North America have been Amerindian. The L/P ratio is normal. The plasma alanine and proline levels are elevated. Urinary organic acid analysis shows elevated concentrations of lactate and 2-ketoglutarate. Patients with the severe form of PC deficiency (type B) present in the newborn period with persistently severe lactic acidosis culminating in death within a few months. In contrast to type A patients, the L/P ratio is elevated. In addition to the biochemical abnormalities found in type A disease, affected infants are mod- erately hyperammonemic, and the concentrations of citrulline, lysine, and proline are increased in plasma. The diagnosis is confirmed by measuring PC activity in peripheral blood leukocytes or in fibroblasts.
Multiple carboxylase deficiency Multiple carboxylase deficiency, either because of holocarboxylase synthetase defi- ciency or biotinidase deficiency, is associated with lactic acidosis, which is the result of deficiency of PC, one of the four biotin-dependent enzymes affected in the disease. Holocarboxylase synthetase deficiency is rare and usually presents within the first few weeks of birth with signs of acute metabolic acidosis accom- panied by hyperammonemia. Feeding problems, failure to thrive, hypotonia, psychomotor retardation, peculiar odor, and seizures are also common and prominent features of the disease. Biotinidase deficiency is more common than 100 A Clinical Guide to Inherited Metabolic Diseases
Table 3.3 Urinary organic acids in multiple carboxylase deficiency
Enzyme deficiency
3-Methylcrotonyl-CoA Propionyl-CoA Pyruvate carboxylase carboxylase carboxylase
3-Methylcrotonate Propionate Lactate 3-Methylcrotonylglycine 3-Hydroxypropionate 3-Hydroxybutyrate 3-Hydroxyisovalerate Methylcitrate Acetoacetate Tiglylglycine
Note: Bold type indicates those compounds that are usually present or present in high concentrations in the disease.
holocarboxylase synthetase deficiency, and clinical presentation is generally later in infancy. Presentation is usually with psychomotor delay, hypotonia, myoclonic seizures, and acute metabolic acidosis. Most patients also have a seborrheic skin rash and at least partial alopecia; many have conjunctivitis, fungal infections, and other evidence of impaired resistance to infection. Some show evidence of optic atrophy, sensorineural hearing loss, and ataxia. The urinary organic acid profile in these disorders reflects the deficiencies of the three biotin-dependent mitochondrial carboxylases involved (Table 3.3). The organic aciduria is variable, particularly in biotinidase deficiency, in which the organic acid pattern in urine may be normal. The diagnosis of biotinidase defi- ciency can be confirmed by enzyme assay on dried blood spots using synthetic chromogenic or fluorogenic substrates; the determination of holocarboxylase syn- thetase is based on the effect of biotin treatment on the activity of the mitochondrial carboxylases in peripheral blood leukocytes or cultured fibroblasts. Both forms of multiple carboxylase deficiency respond to treatment with large doses of oral biotin, 10–20 mg per day, though the response and ultimate outcome tends to be better for infants with holocarboxylase synthetase deficiency.
NADH accumulation
NADH production, like pyruvate production, is increased by any process that increases glycolytic flux. Ignoring for the moment problems of intracellular com- partmentation and the complex matter of NADH transport within the cell, the principal route of NADH disposal by oxidation is by intramitochondrial electron transport linked to ATP generation – the main energy-producing process in the body. In this process, the final electron acceptor is oxygen, and any condition caus- ing local or systemic hypoxia will cause NADH accumulation and lactic acidosis. 101 Metabolic acidosis
NADH accumulation, whether the result of increased production or decreased oxidation, causes lactic acidosis by pushing the pyruvate-lactate equilibrium toward lactate production. Therefore, defectsofNADH oxidation, including inborn errors of the mitochondrial electron transport chain (ETC), are typically characterized by increased L/P ratios in blood and CSF. The laboratory investigation of mitochon- drial defects is discussed in Chapter 9. Arapidly growing number of patients with disease caused by ETC defects is being reported. Although many are associated with lactic acidosis as a result of NADH accumulation, the acidosis is generally not severe and is rarely the prob- lem that brings the patient to medical attention. Instead, most present with one of more of: psychomotor retardation, skeletal myopathy, cardiomyopathy, hepato- cellular dysfunction, or retinal degeneration, although other conditions have been associated with mitochondrial mutations, including diabetes mellitus and other endocrinopathies (see Chapter 2). There is considerable overlap in the relation- ship between the type of mutation, or the ETC complex affected, and the clinical pattern of disease among patients with mitochondrial ETC defects. For example, Leigh disease has been found associated with defects in Complex I, Complex IV, Complex V, as well as PC deficiency and PDH deficiency. In this situation, the clinical presentation provides very little insight into the nature of the underlying genetic defect.
Ketoacidosis
Increased fatty acid oxidation results in the production of large amounts of acetyl- CoA (see Chapter 4). Excess acetyl-CoA is converted in the liver to ketones (3-hydroxybutyrate and acetoacetate) which are transported via the circulation to be taken up and oxidized by peripheral tissues, including the brain (Figure 3.3). This is one of the most important adaptations to starvation because the ability of tissues, such as the brain, which normally derive much of their energy from glucose oxidation, to utilize ketones for energy, spares the glucose for use by tissues, such as erythrocytes, which cannot derive energy from non-glucose energy substrates. Defects in ketone utilization cause ketoacidosis. Ketoacidosis,sometimessevere,isaprominentsecondaryphenomenoninseveral inherited metabolic diseases, such as MSUD, organic acidopathies (e.g., methyl- malonic acidemia, propionic acidemia, isovaleric acidemia, holocarboxylase syn- thetase deficiency), glycogen storage diseases (e.g., GSD type III, hepatic phospho- rylase deficiency, phosphorylase kinase deficiency, glycogen synthase deficiency), and disorders of gluconeogenesis (e.g., pyruvate carboxylase deficiency, fructose-1, 6-diphosphatase deficiency, phosphoenolpyruvate carboxykinase deficiency). Primary disorders of ketone utilization are rare. 102 A Clinical Guide to Inherited Metabolic Diseases
Figure 3.3 Summary of ketone metabolism. The reactions involved in ketone production and oxidation are: 1, acetoacetyl-CoA thiolase; 2, 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthetase; 3, HMG-CoA lyase; 4, succinyl- CoA:3-ketoacid CoA transferase (SCOT).
Mitochondrial acetoacetyl-CoA thiolase deficiency ( -ketothiolase deficiency) -Ketothiolase deficiency is characterized by the onset between one and two years of age of episodic attacks of severe ketoacidosis and encephalopathy, generally precipitated by intercurrent illness or fasting, sometimes associated with hyperam- monemia. The response to treatment with intravenous glucose is characteristically brisk, and between episodes of metabolic decompensation, patients are typically completely well. Urinary organic acid analysis at the time of metabolic decompen- sation shows the presence of 2-methyl-3-hydroxybutyrate, 2-methylacetoacetate, 2-butanone, and tiglylglycine – all derived from the intermediary metabolism of 103 Metabolic acidosis
Figure 3.4 Branched-chain amino acid metabolism. The various enzymes shown are: 1, branched-chain 2-ketoacid decarboxylase; 2, isovaleryl- CoA dehydrogenase; 3, 3-methylcrotonyl-CoA carboxylase; 4, 3-hydroxyl-3-methylglutaryl- CoA (HMG-CoA) synthetase; 5, HMG-CoA lyase; 6, 2-methylacetoacetate thiolase; 7, 3-hydroxyisobutyryl-CoA deacylase; 8, methylmalonyl semialdehyde dehydrogenase; 9, propionyl-CoA carboxylase; 10. methylmalonyl-CoA mutase.
isoleucine (Figure 3.4), as well as huge amounts of 3-hydroxybutyrate and ace- toacetate. Definitive diagnosis requires demonstration of specific deficiency of potassium-stimulated enzyme activity, preferably using 2-methylacetoacetyl-CoA as substrate. Cytosolic acetoacetyl-CoA thiolase deficiency is very rare. It is charac- terizedbysevere psychomotor retardation and hypotonia, a reflection perhaps of the importance of the enzyme in sterol and isoprenoid biosynthesis.
Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency Only a handful of patients with SCOT deficiency have ever been studied in detail. All presented early in life with life-threatening bouts of severe ketoacidosis. Unlike 104 A Clinical Guide to Inherited Metabolic Diseases
patients with -ketothiolase deficiency, who may be to all appearances completely normal between episodes of ketoacidosis, patients with SCOT deficiency are persis- tently ketotic between episodes of metabolic decompensation. The urinary organic acid analysis shows large amounts of 3-hydroxybutyrate and acetoacetate.
Organic aciduria
The development of rapid, accurate, and technically relatively easy and inexpensive techniques for the analysis of low molecular weight organic acids in physiologic flu- ids, like urine, has led to the discovery of a large number of new inherited metabolic diseases.Anumberofthesepresentasacute,chronic,oracute-on-chronicmetabolic acidosis, and urinary organic acid analysis is a logical and important aspect of the diagnostic investigation. However, for some organic acidopathies, clinical signs of metabolic acidosis may be so subtle that they are completely obscured by symptoms referable to the nervous system, heart, liver, kidneys, or other systems. To limit the application of organic acid analysis to patients who have frank metabolic acidosis with increased anion gaps would invariably miss patients affected with some of these disorders. The clinical spectrum of the known disorders of organic acid metabolism spans awide range of presentations involving almost every system in the body. In many cases, the urinary organic acid profile is typical of the disease, and diagnosis is relatively easy. In others, the abnormalities may be quite subtle or only present intermittently. Table 3.4 presents disorders of organic acid metabolism organized according to the principal pathologic urinary organic acid abnormalities. The clin- ical aspects of many of the conditions listed are discussed in other parts of the book more appropriate to the nature of the clinical presentation. Some of the conditions merit specific discussion either because they are relatively common, the interpretation of the clinical and laboratory findings may be difficult, or they serve to illustrate some general principle.
Methylmalonic acidemia (MMA) MMA is a relatively common disorder of organic acid metabolism. However, the metabolism of methylmalonic acid is complex, involving the interaction of a num- ber of distinct gene products and environmental factors (Figure 3.5). Deficiency or adefect in any one of them might produce methylmalonate accumulation. Classi- cal MMA is caused by complete deficiency of methylmalonyl-CoA mutase (mut), which normally catalyzes the rearrangement of methylmalonyl-CoA to succinyl- CoA. It commonly presents in the newborn period in a manner clinically indistin- guishable from propionic acidemia (see Chapter 7)with severe metabolic acido- sis, acute encephalopathy, hyperammonemia, neutropenia, and thrombocytopenia. Table 3.4 Organic acidurias
Urinary organic acids Enzyme defect Distinguishing clinical features
2-Ketoisocaproate, 2-hydroxyisocaproate, 2-keto-3-methylvalerate, Branched-chain 2-ketoacid MSUD: Acute encephalopathy, ketosis, psychomotor retardation 2-ketoisovalerate, 2-hydroxyisovalerate decarboxylase Lactate, 2-ketoglutarate, 2-ketoisocaproate, 2-hydroxyisocaproate, Lipoamide dehydrogenase Psychomotor retardation, chronic lactic acidosis with acute 2-keto-3-methylvalerate, 2-ketoisovalerate, 2-hydroxyisovalerate exacerbations 3-Methylglutaconate, 3-hydroxyisovalerate, 3-methylglutarate, 3-Methylglutaconyl- 3-Methylglutaconic aciduria, type I: mild psychomotor retardation, 3-hydroxybutyrate, acetoacetate CoAhydratase hypoglycemia, ketoacidosis 3-Methylglutaconate, 3-methylglutarate, 2-ethylhydracrylate Tafazzin (TAZ) Barth syndrome (3-methylglutaconic aciduria, type II): X-linked cardiomyopathy, skeletal myopathy, chronic neutropenia 3-Methylglutaconate, 3-methylglutarate Unknown Costeff optic atrophy syndrome: (3methylglutaconic aciduria, type III): optic atrophy, severe psychomotor retardation, choreoathetosis, spasticity, seizures 3-Methylglutaconate, 3-methylglutarate, lactate, TCA cycle Mitochondrial ATP-synthase 3-Methylglutaconic aciduria, type IV: severe multi-organ disease, intermediates congenital malformations, clinically heterogeneous, including Pearson mitochondrial DNA deletion syndrome 3-Hydroxy-3-methylglutarate, 3-methylglutaconate, HMG-CoA lyase Episodic severe metabolic acidosis with encephalopathy, 3-methylglutarate, 3-hydroxyisovalerate hypoglycemia ± hyperammonemia Mevalonate Mevalonate kinase Psychomotor retardation, dysmorphism, cataracts, hepatosplenomegaly, lymphadenopathy, anemia, chronic diarrhea, arthralgia, fever, skin rash Isovalerylglycine, 3-hydroxyisovalerate, lactate, 3-hydroxybutyrate, Isovaleryl-CoA Severe metabolic acidosis, hyperammonemia, neutropenia, acetoacetate dehydrogenase thrombocytopenia, odor of sweaty feet 2-Methyl-3-hydroxybutyrate, 2-methylacetaoacetate, 2-butanone, Mitochondrial Episodic severe ketoacidosis 3-hydroxybutyrate, acetoacetate, tiglylglycine acetoacetyl-CoA thiolase 3-Methylcrotonate, 3-methylcrotonylglycine, 3-hydroxyisovalerate 3-Methylcrotonyl-CoA Episodic severe ketoacidosis, hypoglycemia carboxylase 3-Methylcrotonate, 3-methylcrotonylglycine, 3-hydroxyisovalerate, (a) Holocarboxylase (a) Metabolic acidosis, hyperammonemia, thrombocytopenia, propionate, 3-hydroxypropionate, methylcitrate, tiglylglycine, synthetase or peculiar odor, seizures, ataxia, (skin rash, alopecia) lactate, 3-hydroxybutyrate and acetoacetate (b) Biotinidase (b) Psychomotor delay, hypotonia, myoclonic seizures, metabolic acidosis, seborrheic skin rash, alopecia Ethylmalonate, methylsuccinate, butyrylglycine, isovalerylglycine, A mitochondrial matrix Ethylmalonic encephalopathy Spastic diplegia, orthostatic 2-methylbutyrylglycine protein (ETHE1) with acrocyanosis, chronic diarrhea, psychomotor retardation, lactic uncertain function acidosis Ethylmalonate, isobutyrylglycine, isovalerylglycine, Cytochrome c oxidase Psychomotor retardation, encephalopathy, ataxia, spasticity 2-methylbutyrylglycine l-2-Hydroxyglutarate Unknown Ataxia, dysarthria, psychomotor retardation, ± seizures d-2-Hydroxyglutarate d-2-Hydroxyglutarate Psychomotor retardation, seizures dehydrogenase Methylmalonate, methylcitrate, 3-hydroxybutyrate, acetoacetate Methylmalonyl-CoA mutase Severe metabolic acidosis, hyperammonemia, neutropenia, or Cobalamin defects thrombocytopenia 3-Hydroxyisobutyrate, lactate 3-Hydroxyisobutyryl-CoA Episodic ketoacidosis, facial dysmorphism, cerebral dysgenesis, dehydrogenase hypotonia, failure to thrive 4-Hydroxybutyrate, 3,4-dihydroxybutyrate Succinic semialdehyde Psychomotor retardation, hypotonia, ataxia, choreoathetosis dehydrogenase Fumarate Fumarase Psychomotor retardation Propionate, 3-Hydroxypropionate, propionylglycine, methylcitrate, Propionyl-CoA carboxylase Severe metabolic acidosis, hyperammonemia, neutropenia, tiglylglycine, 3-hydroxybutyrate, acetoacetate thrombocytopenia Malonate Malonyl-CoA decarboxylase Psychomotor retardation ± cardiomyopathy l-Glycerate, oxalate d-Glycerate dehydrogenase Urolithiasis, urinary tract infections, renal colic Oxalate, glycolate Alanine: glyoxylate Urolithiasis, nephrocalcinosis, peripheral neuropathy, anemia, aminotransferase (type I) arthropathy, progressive renal failure Medium-chain dicarboxylic acids (adipate, suberate, sebacate), Medium-chain acyl-CoA Recurrent Reye-like encephalopathy, sudden unexpected death 5-hydroxyhexanoate, 7-hydroxyoctanoate, hexanoylglycine, dehydrogenase phenylpropionylglycine, octanoylcarnitine (cont.) Table 3.4 (cont.)
Urinary organic acids Enzyme defect Distinguishing clinical features
Ethylmalonate, methylsuccinate, adipate, butyrylglycine Short-chain acyl-CoA Skeletal myopathy, cardiomyopathy, failure to thrive, metabolic dehydrogenase acidosis Medium-chain dicarboxylic acids, dodecanedioate, Long-chain acyl-CoA Cardiomyopathy, skeletal myopathy, exercise intolerance with tetradecanedioate dehydrogenase myoglobinuria, Reye-like episodes of acute encephalopathy Medium-chain dicarboxylic acids, 3-hydroxydodecanedioate, Trifunctional protein Cardiomyopathy, variable skeletal myopathy, intermittent acute 3-hydroxydodecenedioate, 3-hydroxytetradecanedioate, (long-chain hepatocellular dysfunction, peripheral neuropathy 3-hydroxytetradecenedioate 3-hydroxyacyl-CoA dehydrogenase) Glutarate, 3-hydroxyglutarate Glutaryl-CoA Progressive dystonia, choreoathetosis, intermittent ketoacidosis and dehydrogenase acute encephalopathy Glutarate, 2-hydroxyglutarate, ethylmalonate, adipate, suberate, Electron transfer flavoprotein Severe: Facial dysmorphism, cerebral dysgenesis, cystic kidneys, or sebacate, dodecanedioate, isovalerylglycine, hexanoylglycine (ETF) or ETF Mild: Intermittent severe ketoacidosis, hyperammonemia, acute dehydrogenase encephalopathy, failure to thrive 5-Oxoproline (pyroglutamate) Glutathione synthetase Hemolytic hypochromic, microcytic anemia 4-Hydroxycyclohexylacetate 4-Hydroxyphenyl- Hawkinsinuria: autosomal dominant intermittent metabolic pyruvate oxidase acidosis in infancy N-Acetylaspartate Aspartoacylase Canavan syndrome: Severe, progressive psychomotor retardation, macrocephaly, seizures Orotate (a) UMP synthase or (a) Megaloblastic anemia, urolithiasis, failure to thrive, (b) Various defects in urea psychomotor retardation biosynthesis (b) See Chapter 2 Uracil, thymine Dihydropyrimidine Uncertain. Increased susceptibility to 5-fluorouracil toxicity dehydrogenase
Abbreviations: MSUD, maple syrup urine disease; TCA, tricarboxylic acid; HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA; ETF, electron transfer flavoprotein. 108 A Clinical Guide to Inherited Metabolic Diseases
Figure 3.5 Relationship between cobalamin, methylmalonic acid (MMA), and homocysteine metabolism. The letters, A to F, refer to the locations of the metabolic defects of the different complementation groups of inherited defects in cobalamin metabolism. Abbreviations: Cbl, cobalamin; TC II, transcobalamin II; MS-Cbl, methionine synthase-bound cobalamin; MMACoA, methylmalonyl-CoA; SuccCoA, succinyl-CoA; AdoCbl, adenosylcobalamin; mut, methylmalonyl-CoA mutase.
Late-onset variants of the disease, in which residual mutase activity is high, are common. Generally, the later the onset, the milder the disease; some individuals with MMA as a result of mut mutations show no symptoms at all. Methylmalonyl-CoA mutase is one of only two human enzymes known to
require cobalamin (vitamin B12) for activity. MMA caused by defects in the 109 Metabolic acidosis
intramitochondrial processing or adenosylation of cobalamin (cblA and cblB vari- ants, respectively) and defects affecting the affinity of mutase for adenosylcobal- amin, is often somewhat milder than disease caused by complete mutase deficiency,
and it is often responsive to treatment with pharmacologic doses of vitamin B12.In every other respect, it is clinically indistinguishable from classical MMA. Methionine synthase (MS) is the other enzyme in the body that requires cobal- amin for activity. In this case, the active species of the cofactor is methylcobalamin. Defects in the processing of MS-Cbl (cblE and cblG variants) cause homocystinemia and homocystinuria, but not MMA. Patients with cblE or cblG disease present early in life with psychomotor retardation, feeding difficulties and failure to thrive, hypo- tonia, cerebral atrophy, and megaloblastic anemia that is hematologically indistin-
guishable from that caused by nutritional vitamin B12 deficiency. In contrast to the marked elevation of plasma methionine concentrations in classical homocystinuria due to cystathionine -synthase deficiency, the methionine levels in patients with cblE or cblG defects are, as one would expect, decreased below normal. Defects in the transport, intracellular uptake, lysosomal processing, release from lysosomes, or reduction of Cbl+++ to Cbl++ are characterized biochemically by both MMA and homocyst(e)inemia and homocystinuria. All these defects are associated with megaloblastic anemia, variable psychomotor retardation, and fail- ure to thrive, some with onset in early infancy and others only emerging in later life. Patients with hereditary defects in cobalamin processing (cblC, cblD and cblF variants) generally have more severe disease than those with defects in cobalamin absorption and transport (e.g., transcobalamin II deficiency). Developmental retar- dation, failure to thrive, seizures, and megaloblastic anemia are prominent, along with MMA and homocystinuria. Although symptoms of feeding difficulty and hypotonia often develop in the first few weeks of life (especially in cblC disease), urinary methylmalonic acid levels are never as high as in MMA due to mut defi- ciency, and acute metabolic acidosis with hyperammonemia does not occur, even in patients with early-onset variants of these cobalamin defects. The clinical vari- ability among patients with different cobalamin defects is considerable, making classification of the defects on clinical grounds alone unreliable. As a rule, defini- tive classification requires complementation studies on cultured skin fibroblasts (see Chapter 9). Over the years, we have encountered a number of infants presenting in the first few months of life with MMA, megaloblastic anemia, and homocystinuria with
normal or low plasma methionine levels, as a result of dietary vitamin B12 defi- ciency. In every case, the mother was a strict vegan and the infant was breast fed. The cause of the metabolic abnormalities in each case was confirmed by demon-
strating that plasma vitamin B12 levels were well below normal. We have also seen abreast-fed infant who presented at 6 months of age with a history of marked 110 A Clinical Guide to Inherited Metabolic Diseases
developmental delay, hypotonia, lethargy, and methylmalonic aciduria associated
with vitamin B12 deficiency caused by previously unrecognized maternal pernicious anemia.
3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase deficiency HMG-CoA lyase catalyzes the last step in the intramitochondrial catabolism of the amino acid, leucine (Figure 3.4). The products of the reaction, acetoacetate and acetyl-CoA, are important energy substrates, particularly during illness or fasting. Patients with HMG-CoA lyase deficiency may present in the newborn period, in a manner resembling neonatal propionic acidemia or MMA, with severe metabolic acidosis, vomiting, lethargy and drowsiness progressing to coma, poor feeding, hypoglycemia in most, and hyperammonemia in many. However, HMG-CoA lyase deficiency is different from the other organic acidopathies pre- senting with similar symptoms because of the absence of ketonuria. The dis- ease presenting for the first time in older infants often resembles Reye syndrome or a fatty acid oxidation defect, such as medium-chain acyl-CoA dehydroge- nase (MCAD) deficiency (see Chapter 2). The findings of enlargement of the liver and abnormal liver function tests add to the potential for diagnostic con- fusion. However, urinary organic acid analysis characteristically shows abnormal- ities typical of the disease, particularly during metabolic crises: massive excretion of 3-hydroxy-3-methylglutarate and 3-methylglutaconate, and large amounts of 3-hydroxyisovalerate and 3-methylglutarate. Lactic acidosis and marked increases in urinary glutaric acid and adipic acid levels are often seen during very severe metabolic crises. Plasma carnitine levels are decreased and the proportion of esteri- fied carnitine is increased as a result of the formation of 3-methylglutarylcarnitine; neither HMG or 3-methylglutaconate form carnitine esters in patients with this disease. Treatment is effective in decreasing the frequency and severity of episodes of acute metabolic decompensation. In spite ofapparently adequate treatment, with a high carbohydrate, low protein diet supplemented with carnitine, some patients develop cardiomyopathy, which may be fatal.
Glutaric aciduria Glutaric aciduria type I (GA I), caused by deficiency of mitochondrial glutaryl- CoAdehydrogenase, usually presents in early infancy as a neurologic syndrome (see Chapter 2). After some weeks or months of apparently normal development, affected infants suddenly develop the first of recurrent episodes of marked hypo- tonia, dystonia, opisthotonus, grimacing, fisting, tongue thrusting and seizures. Partial recovery is followed by progressive neurologic deterioration and periodic episodes of ketoacidosis, vomiting and acute encephalopathy, usually precipi- tated by intercurrent infections. In some patients, the neurologic abnormalities 111 Metabolic acidosis
Table 3.5 Flavoprotein dehydrogenases for which ETF/ETF dehydrogenase is the electron acceptor
Mitochondrial fatty acid -oxidation Very long-chain acyl-CoA dehydrogenase Long-chain acyl-CoA dehydrogenase Medium-chain acyl-CoA dehydrogenase Short-chain acyl-CoA dehydrogenase Leucine oxidation Isovaleryl-CoA dehydrogenase Valine and isoleucine oxidation 2-Methylbutyryl-CoA dehydrogenase Lysine, hydroxylysine, and tryptophan oxidation Glutaryl-CoA dehydrogenase Choline oxidation Dimethylglycine dehydrogenase Sarcosine dehydrogenase
Abbreviations: ETF, electron transfer flavoprotein.
remain relatively stationary with gross motor retardation, chronic choreoathetosis, dystonia,andhypotonia,withapparentpreservationofintellect.CNSimagingstud- ies typically show early cortical atrophy and attenuation of white matter and basal ganglia (see Figure 2.14). Some patients present with acute Reye-like disease without the extrapyramidal neurologic signs. During acute metabolic decom- pensation, laboratory studies show metabolic acidosis and ketosis, hypoglycemia, hyperammonemia, and mild hepatocellular dysfunction. Besides marked increases in glutaric acid concentration, urinary organic acid analysis shows the presence of 3-hydroxyglutarate, considered pathognomonic of the disease, and sometimes glutaconic acid during severe ketoacidosis. Between episodes of metabolic decom- pensation, the urinary organic acids may be normal or only mildly abnormal. Plasma carnitine levels are decreased. Glutaric aciduria type II (GA II), which is also called multiple acyl-CoA dehydro- genase deficiency, is caused by deficiency of either electron transport flavoprotein (ETF), the intramitochondrial electron acceptor for a number of acyl-CoA dehy- drogenases, or ETF dehydrogenase (Table 3.5). The condition may present in one of three ways: r Very severe, neonatal disease, characterized by facial dysmorphism, muscular defects of the abdominal wall, hypospadias (in males), cystic disease of the kid- neys, hypotonia, hepatomegaly, hypoketotic hypoglycemia, metabolic acidosis, and hyperammonemia (see Chapters 6 and 7). 112 A Clinical Guide to Inherited Metabolic Diseases
r Severe neonatal disease without dysmorphism, but with hypotonia, hepato- megaly, hypoketotic hypoglycemia, metabolic acidosis, and hyperammonemia. r Mild disease characterized by later-onset episodic acute metabolic acidosis, failure to thrive, hypoglycemia, hyperammonemia, and encephalopathy. The severe variants are often associated with a peculiar odor of sweaty feet similar to that encountered in infants with severe isovaleric acidemia. Plasma amino acid analysis shows elevations of several amino acids, especially proline and hydrox- yproline. Urinary organic acid analysis in infants with severe variants of the dis- ease characteristically shows very large amounts of glutarate, ethylmalonate, and the dicarboxylic acids, adipate, suberate, and sebacate, in addition to isovalerate, isovalerylglycine, 2-hydroxyglutaraate, hexanoylglycine, and 5-hydroxyhexanoate. The mild form of GA II is often called ethylmalonic-adipic aciduria, referring to the predominant urinary organic acid abnormalities. However, the urinary organic acids may be normal between episodesofmetabolic decompensation. Secondary glutaric aciduria is much more common than glutaric aciduria due to primary disorders of glutaric acid metabolism, like GA I and GA II. It is commonly found in relatively large concentrations in infants with mitochondrial ETC defects, presumably a reflection of ‘sick mitochondria’. We have also seen massive glutaric aciduria in a boy with late-onset, but severely decompensated propionic acidemia. It has been reported in 2-ketoadipic aciduria ( -aminoadipic acidemia), probably as a result of nonenzymic decarboxylation of 2-ketoadipate. It is also one of the dicarboxylic acids appearing in the urine of infants on medium-chain triglyceride formulas.
Dicarboxylic aciduria Increased concentrations of the medium-chain dicarboxylic acids, adipic (6- carbon), suberic (8-carbon), and sebacic (10-carbon) acids, is one of the most prominent laboratory abnormalities in patients with inherited disorders of mito- chondrial fatty acid -oxidation, such as medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. These disorders usually present as acute neurologic or hepatic syndromes, rather than as metabolic acidosis (see Chapters 2 and 4). Medium-chain dicarboxylic aciduria is also a common secondary feature of several other conditions. The levels of adipic, suberic, and sebacic acids in the urine are generally increased under any circumstances in which fatty acid utilization is increased beyond the capacity for mitochondrial -oxidation, such as during starvation and in patients with diabetesmellitus. It is also commonlyseen in patients on the anticonvulsant, valproic acid, which inhibits fatty acid -oxidation, and in newborn infants. When the dicarboxylic aciduria is the result of increased fatty acid oxidation, it is routinely associated with marked ketosis and the excretion of large amounts of 3-hydroxybutyrate (3-HOB) and acetoacetate. The ratio of 3-HOB to 113 Metabolic acidosis
adipate is generally >2.0. By contrast, in patients with mitochondrial fatty acid -oxidation defects, the urinary ketone concentrations are characteristically low, and the 3-HOB/adipate ratio is <2.0. Unfortunately, in very young infants, or in infants on formulas containing medium-chain triglycerides, the relationship breaks down; many apparently healthy newborn infants, particularly low birth-weight prematureinfants,excreteamountsofmediumchaindicarboxylicacidscomparable to the levels seen in asymptomatic infants with fatty acid oxidation defects. Analyses of urinary acylglycines and acylcarnitines, and especially measurements of plasma acylcarnitines by tandem MSMS, are particularly helpful in distinguishing infants with genetic defects in fatty acid metabolism.
Ethylmalonic aciduria Ethylmalonate and adipate are particularly prominent in the urine of patients with the mild variant of multiple acyl-CoA dehydrogenase deficiency (GA II). However, ethylmalonate is excreted in a wide variety of other circumstances, some associated with severe systemic inherited metabolic diseases, others being quite benign. Increased concentrations of ethylmalonate, along with methylsuccinate, may be the only urinary organic abnormalities in patients with short-chain acyl-CoA dehydrogenase (SCAD) deficiency. Ethylmalonic aciduria, without methylsucci- nate, is also found in patients with cytochrome c oxidase deficiency. The combi- nation of lactic acidosis, ethylmalonic, and methylsuccinic aciduria, along with the excretion of butyrylglycine, isovalerylglycine, and 2-methylbutyrylglycine, is characteristic of ethylmalonic encephalopathy, characterized clinically by severe psychomotor retardation, spasticity, chronic diarrhea, petechiae, and orthostatic acrocyanosis. The disease is caused by mutations in ETHE1, the gene coding for a mitochondrial matrix protein of uncertain function.
D-Lactic acidosis Infants or young children with gastrointestinal abnormalities, such as blind loops, involving bowel stasis, sometimes develop attacks of severe metabolic acidosis, often associated with acute encephalopathy, with increased anion gap. Plasma lactate, 3-hydroxybutyrate, and acetoacetate levels may be completely normal. However, organic acid analysis shows the presence of large amounts of lactic acid in the urine. The acidosis in these cases is not the result of an inborn error of metabolism; it is caused by accumulation of d-lactate, a product of bacterial carbohydrate metabolism that is readily absorbed from the gut. The routine measurement of lactate in blood is by an enzymic method, employing LDH, which is specific for l-lactate, the usual product of carbohydrate metabolism in humans. Urinary organic acid analysis is generally carried out by chromatographic techniques, such as gas chromatography-mass spectrometry (GC-MS), which do not differentiate the 114 A Clinical Guide to Inherited Metabolic Diseases
Table 3.6 Some common causes of spurious or artefactual organic aciduria
Organic acid Underlying condition or disease
d-Lactic acid Intestinal bacterial over-growth. May be sufficient to cause metabolic acidosis and encephalopathy Methylmalonic, ethylmalonic, and Very young infants with gastroenteritis; may be 3-hydrocypropionic acids associated with methemoglobinemia Medium-chain dicarboxylic acids Valproic acid administration. The pattern often (adipic>suberic>sebacic) resembles that seen in patients with defects in mitochondrial fatty acid oxidation. Medium-chain dicarboxylic acids Ingestion of formulas containing medium-chain (sebacic>suberic>adipic) triglycerides. The relationship between the organic acids varies according to the fatty acid composition of the medium-chain triglyceride. Adipic acid Ingestion of large amounts of Jello® containing adipic acid additive. The elevation of adipate may be large, but the absence of any other organic acid abnormality suggests the underlying dietary etiology. Long-chain 3-hydroxydicarboxylic Acetaminophen intoxication; severe hepatocellular acids disease. Pivalic acid Pivampicillin or pivmecillinam administration. Octenylsuccinc acid Formulas containing octenylsuccinate-modified cornstarch as emulsifying agent.
Methylmalonic acid A prominent feature of dietary vitamin B12 deficiency. Azelaic and pimelic acids Extracts from plastic storage containers. 2-Hydroxybutyric acid Occurs with severe lactic acidosis, irrespective of the cause. 5-Oxoproline (pyroglutamic acid) Acetaminophen or vigabatrin ingestion.
d-isomer of lactate from the l-isomer. The marked discrepancy between the results of lactate measurements by the two techniques provides the clue to the origin of the acidosis. Treatment with oral, non-absorbed, antimicrobials usually produces rapid resolution of the acidosis, though recurrence of the problem is common.
Adventitious organic aciduria In addition to expanding tremendously the number of identified inherited metabolic diseases, the widespread application of urinary organic acid analysis has also posed some challenging problems in interpretation owing to the effects of age, bowel flora, intercurrent illness, and medications, on urinary organic acid excretion. Some of the more common causes of urinary organic artifacts are shown in Table 3.6. 115 Metabolic acidosis
SUGGESTED READING
Fall, P. J. (2000). A stepwise approach to acid-base disorders. Practical patient evaluation for metabolic acidosis and other conditions. Postgraduate Medicine, 107, 249–50, 253–4, 257–8 passim. Goldstein, M. B., Bear, R., Richardson, R. M. A., Marsden, P. A. & Halperin, M. L. (1986). The urine anion gap: a clinically useful index of ammonium excretion. American Journal of Medical Science, 292, 198–202. Lehotay, D. & Clarke, J. T. R. (1995). Organic acidurias and related abnormalities. Critical Reviews in Clinical Laboratory Sciences, 32, 377–429. Mitchell, G. A., Kassovska-Bratinova, S., Boukaftane, Y. et al. (1995). Medical aspects of ketone body metabolism. Clinical and Investigative Medicine, 18, 193–216. Niaudet, P. & Rotig, A. (1996). Renal involvement in mitochondrial cytopathies. Pediatric Nephrology, 10, 368–73. Ogier de Baulny, H. & Saudubray, J. M. (2002). Branched-chain organic acidurias. Seminars in Neonatology, 7, 65–74. Rabier, D., Bardet, J., Parvy, Ph. et al. (1995). Do criteria exist from urinary organic acids to distinguish β-oxidation defects? Journal of Inherited Metabolic Diseases, 18, 257–60. Rotig, A. (2003). Renal disease and mitochondrial genetics. Journal of Nephrology, 16, 286–92. Stacpoole, P. W. (1997). Lactic acidosis and other mitochondrial disorders. Metabolism, 46, 306–21. Thorburn, D. R. & Dahl, H. H. (2001). Mitochondrial disorders: genetics, counseling, prenatal diagnosis and reproductive options. American Journal of Medical Genetics, 106, 102–14. Uribarri, J., Oh, M. S. & Carroll, H. J. (1998). D-lactic acidosis. A review of clinical presentation, biochemical features, and pathophysiologic mechanisms. Medicine (Baltimore), 77, 73–82. 4 4 Hepatic syndrome
Liver involvement of some kind is a presenting feature of a number of inherited metabolic diseases. The metabolic activities of the liver span a vast catalogue of func- tions important to the metabolism of the entire body. It is surprising, therefore, that the repertoire of responses to injury is limited, and inborn errors of metabolism manifesting as hepatic syndrome are commonly difficult to distinguish from many acquired conditions, such as infections, intoxications, developmental abnormali- ties, and neoplasia. One approach to the diagnosis of inherited metabolic diseases presenting as hepatic syndrome is to consider four possible presentations, recog- nizing that there is considerable overlap between them. They are: r jaundice; r hepatomegaly; r hypoglycemia; r hepatocellular dysfunction.
Jaundice
Jaundice is caused by accumulation of unconjugated or conjugated bilirubin, which may occur as a result of increased production, impaired metabolism, or biliary obstruction. Bilirubin is a porphyrin pigment derived from the degradative metabolism of the heme of hemoglobin.
Unconjugated hyperbilirubinemia Pure unconjugated hyperbilirubinemia is characteristic of disorders associated with increased bilirubin production. Mature erythrocytes have no mitochondria. They derive virtually all the energy needed to maintain ion gradients, intracel- lular nucleotide concentrations, membrane plasticity, the iron of hemoglobin in the reduced state, and other functions, from glycolysis and the hexose monophos- phate shunt. Not surprisingly, specific hereditary deficiencies of any of the enzymes involved commonly present with hemolytic anemia. Some are also associated with 116 117 Hepatic syndrome
neurologic symptoms, such as severe psychomotor retardation (e.g., triosephos- phate isomerase deficiency) or myopathy (e.g., phosphofructokinase deficiency) (see Chapter 2). The hyperbilirubinemia caused by hemolysis is characteristically unconjugated, and it is not generally accompanied by any clinical or biochemical evidence of hepatocellular dysfunction. The commonestinbornerror or erythrocytemetabolism presentingas jaundiceis X-linked recessive glucose-6-phosphate dehydrogenase (G6PD) deficiency, a defect in the first reaction of the hexose monophosphate shunt. Carriers of the gene show relative resistance to malaria accounting for the high prevalence of the mutation in areas of the world where it is endemic. Acute hemolysis is typically precipitated by intercurrent illness or exposure to oxidizing drugs, such as sulfonamides and antimalarials, though it may occur spontaneously in the newborn period. The com- monest inborn error of glycolysis presenting as unconjugated hyperbilirubinemia is pyruvate kinase (PK) deficiency which, like G6PD deficiency, may present in the newborn period with severe nonspherocytic hemolytic anemia. Unconjugated hyperbilirubinemia is also a feature of some primary disorders of bilirubin metabolism. Normal bilirubin metabolism involves uptake by hepato- cytes, conjugation with glucuronic acid, and excretion in bile. At least some indi- viduals with Gilbert syndrome, a common (3% of the population), benign disorder of bilirubin metabolism associated with mild persistent unconjugated hyperbiliru- binemia, generally presenting after puberty,appear to have a defect in bilirubin uptake along with partial deficiency of bilirubin UDP-glucuronosyltransferase (BGT). The absence of any evidence of hemolysis or hepatocellular dysfunction is typical of this condition. Severe neonatal unconjugated hyperbilirubinemia caused by specific BGT defi- ciency is characteristic of Crigler-Najjar syndrome. It is commonly associated with unconjugated bilirubin levels >500 mol/L in the absence of hemolysis, infection, or significant hepatocellular dysfunction. Phototherapy and exchange transfusions are ineffective, and affected infants invariably develop severe ker- nicterus. Some patients, classified as Crigler-Najjar syndrome type 2 (also called Arias syndrome), respond to administration of phenobarbital (4 mg/kg/day) with adramatic drop in plasma bilirubin levels. Patients with Crigler-Najjar syndrome are not usually difficult to distinguish from patients with breast milk jaundice, which is milder, later in onset, and can be shown to be associated with breast feeding. It is important to remember that the hyperbilirubinemia in infants with classical galactosemia is often initially unconjugated, converting only after a period of some days to the conjugated hyperbilirubinemia that is widely regarded as characteristic of the disease. Even early in the course of the disease, galactosemia is associated with evidence of significant hepatocellular dysfunction, which sets it apart from 118 A Clinical Guide to Inherited Metabolic Diseases
Crigler-Najjar syndrome. Galactosemia is discussed in more detail in the section ‘Hepatocellular dysfunction’.
Conjugated hyperbilirubinemia Conjugated hyperbilirubinemia as a manifestation of inherited metabolic disease is more common than unconjugated hyperbilirubinemia because it includes those diseases, like galactosemia, hepatorenal tyrosinemia, and hereditary fructose intol- erance, in which hepatocellular dysfunction is prominent (see ‘Hepatocellular dys- function’). Mixedconjugated and unconjugated hyperbilirubinemia in the absence of any other evidence of hepatocellular dysfunction or hemolysis, with onset in later child- hood, is typical of Rotor syndrome or Dubin-Johnson syndrome caused by benign defects in the intrahepatic biliary excretion of bilirubin glucuronide. The two con- ditions are differentiated from each other by differences in urinary porphyrins. The former is associated with a marked increase in urinary excretion of coproporphyrin I and III with <80% being the I isomer; in Dubin-Johnson syndrome, the urinary coproporphyrin levels may be normal, but the I isomer accounts for >80% of the total (normal about 25%).
Hepatomegaly
Asymptomatic hepatomegaly is common in children, and the decision about who to investigate, and how intensively, is sometimes difficult. The hepatomegaly asso- ciated with inherited metabolic diseases is generally persistent and nontender. If the liver is so soft that the edge is difficult to palpate, enlargement is likely to be due to accumulation of triglyceride, a typical feature of GSD (glycogen storage dis- ease) type I. At the other extreme, a hard and irregular liver edge, often associated with only modest enlargement of the organ, is characteristic of cirrhosis, such as is characteristic of hepatorenal tyrosinemia (hereditary tyrosinemia, type I). When it is enlarged as a result of lysosomal storage, the liver is usually firm, but not hard. Is the spleen also enlarged? A history of hematemesis or the presence of ascites or abdominal venous dilatation, would suggest that splenomegaly is caused by portal hypertension resulting from cirrhosis. However, the spleen may be enlarged by infiltration or accumulation of the same cells or metabolites causing enlargement of the liver. Besides sharing the portal circulation, the liver and spleen both contain components of the reticuloendothelial system (RES). Conditions causing expansion of the RES, either as a result of cellular proliferation or storage within RES cells (i.e., macrophages), commonly present with clinical enlargement of both organs. This is characteristic, for example, of many of the lysosomal storage diseases (see Chapter 6). 119 Hepatic syndrome
Glycogen storage disease, type III (GSD III) commonly presents as asymptomatic hepatomegaly discovered incidentally in the course of routine physical examination. The spleen may also be enlarged, but the splenomegaly is mild compared with the enlargement of the liver. Glycogen accumulation in this condition is caused by deficiency of a debrancher enzyme that converts the branch-points in glycogen into linear molecules for further hydrolysis by phosphorylase. The enlargement of the liver may be marked. It is generally firm and nontender, with a sharp, smooth, edge that is easy to palpate. In most patients, hypoglycemia does not occur, or it occurs only after prolonged fasting. However, in a significant minority, it may present in early infancy and be as severe as the hypoglycemia seen in patients with GSD I. Severe early infantile GSD III may also be associated with failure to thrive and hyperlipidemia, further blurring the clinical differentiation from GSD I. However, lactic acidosis and hyperuricemia do not occur, or are very mild, in patients with GSD III. Moreover, the condition is associated with ketosis during fasting and with moderate increases in liver aminotransferases (AST and ALT), which, as a rule, do not occur in GSD I. Liver biopsy shows increased glycogen with variable interlobular fibrosis,butverylittlefat.Rarely,thefibrosisprogressestofrankcirrhosis,producing portal hypertension and liver failure. As adults, many patients develop evidence of muscle involvement, including cardiomyopathy in some. This is characterized by proximal muscle weakness, depressed deep tendon reflexes, and elevation of plasma creatine phosphokinase (see Chapter 2). Patients with GSD III will show a rise in plasma glucose in response to ingestion of galactose, fructose, or amino acids, indicating that gluconeogenesis is intact. They also show a significant increase in plasma glucose in response to glucagon admin- istered two to four hours after feeding, but they do not respond after 10–12 hours of fasting when all the hepatic linear glycogen accessible to phosphorylase activ- ity has been depleted. Confirmation of the diagnosis requires measurement of debrancher enzyme activity in fresh liver obtained by biopsy. Hepatic phosphorylase deficiency (GSD VI) is often clinically indistinguishable from GSD III, though it is much less common, and involvement of skeletal mus- cle and the heart does not occur. Phosphorylase deficiency can be demonstrated histochemically on tissue obtained by biopsy. Phosphorylase b kinase deficiency is more common than GSD VI. The most com- mon variant appears to be transmitted as an X-linked recessive disorder. Clinically, it is often indistinguishable from GSD III. However, unlike patients with GSD III, patients with this type of glycogen storage disease show only minimal increases in plasma glucose in response to glucagon after fasting of any duration. Liver biopsy shows increased glycogen, which may be more dispersed in appearance than in GSD III. There is often some interlobular fibrosis, though cirrhosis is rare. Con- firmation of the diagnosis is best done by direct enzyme analysis of fresh liver, 120 A Clinical Guide to Inherited Metabolic Diseases
although some patients show deficiency of the enzyme in red blood cells. Involve- ment of skeletal muscle occurs in a small proportion of patients, in which the condition appears to be transmitted as an autosomal recessive. Isolated involve- ment of skeletal muscle or the myocardium is very rare (see Chapters 2 and 5). Mutation analysis is also often helpful in confirming the diagnosis.
Hypoglycemia
Hunger, apprehension, jitters, irritability, and sweating are common early symp- toms of hypoglycemia in older patients. Unless the cause of the symptoms is recog- nized and treated, this is followed by disturbance of consciousness with drowsiness progressing rapidly to stupor and coma accompanied by convulsions. Idiosyncratic presentations dominated by behavioural abnormalities are common. In very young infants, the early signs may be subtle with nothing more than irritability, sweating, and somnolence. A seizure may be the first recognized indication of the problem, and hypoglycemia should be considered in any infant presenting for the first time with convulsions. Treatment with intravenous glucose should not be delayed. The differential diagnosis of hypoglycemia is made easier by some understanding of the normal mechanisms for maintaining normal plasma glucose concentrations during fasting. During the intervals between meals, the plasma concentration is supported by two general mechanisms: r Mechanisms directed at producing glucose (glycogen breakdown and gluconeogenesis); r Mechanisms that decrease peripheral glucose utilization by providing alternative energy substrates (fatty acid and ketone oxidation). Hypoglycemia may occur as a result of primary or secondary defects in glucose production (deficiency of supply), or as a result of defects in fatty acid or ketone oxidation (over-utilization).
Ways to increase glucose production Glycogen is a high-molecular weight, highly branched polymer of glucose. During feeding it is formed by polymerization of glucose, derived primarily from dietary carbohydrate. During fasting, the process is reversed with glucose being released by phosphorylase-catalyzed hydrolysis of glycogen. Glycogen is an excellent form of immediately available glucose. However, storage in the liver involves the simul- taneous storage of large amounts of water, and the total amount of glycogen that can be accommodated is, therefore, actually relatively small. As a result, within only 24–48 hours of fasting, the glycogen in the liver becomes totally depleted as it is rapidly converted into glucose to meet the needs of tissues, like the brain, having high energy requirements. 121 Hepatic syndrome
Figure 4.1 Overview of key reactions in gluconeogenesis. The various enzymes involved in key reactions of gluconeogensis are: 1, lactate dehydroge- nase (LDH); 2, pyruvate dehydrogenase complex (PDH); 3, pyruvate carboxylase (PC); 4, ala- nine aminotransferase (ALT); 5, phosphoenolpyruvate carboxykinase (PEPCK); 6, glucose- 6-phosphatase.
The synthesis of glucose from nonglucose substrates (gluconeogenesis) occurs coincidentally with glycogenolysis during fasting, and it is ultimately capable of sup- plying much more glucose over a longer period of time. The process (Figure 4.1), which takes place predominantly in the cytosol, is functionally the reverse of glycolysis. One of the most important regulatory steps in the process is the car- boxylation of pyruvate to form oxaloacetate (catalyzed by pyruvate carboxylase) 122 A Clinical Guide to Inherited Metabolic Diseases
within mitochondria. The oxaloacetate formed by the reaction is then converted to phosphoenolpyruvate in a reaction catalyzed by mitochondrial phosphoenolpyru- vate carboxykinase (PEPCK). The PEP diffuses out of the mitochondria into the cytoplasm where it is converted to glucose in a series of reactions that mirror the same steps in glycolysis. Oxaloacetate is also transported out of mitochondria into the cytoplasm by the ‘malate shuttle’. Cytosolic oxaloacetate is converted to PEP by cytosolic PEPCK, which is genetically distinct from the mitochondrial isozyme. There is some evi- dence that mitochondrial PEPCK is particularly important in the synthesis of glu- cose from pyruvate derived from lactate, and that cytosolic PEPCK is more impor- tant in gluconeogenesis involving oxaloacetate and pyruvate derived from amino acid metabolism. Other important gluconeogenic substrates, such as galactose, fructose, and glyc- erol, feed into the process at different steps between PEP and glucose-6-phosphate. The final step in both glycogenolysis and gluconeogenesis is glucose-6-phosphatase- catalyzed hydrolysis of glucose-6-phosphate to form free glucose. Acritical aspect of gluconeogenesis is an adjustment made to preserve and re- utilize the carbon skeleton of glucose, rather than having it lost irretrievably as a
result of oxidation all the way to CO2.This process, which is called the Cori cycle (Figure 4.2), involves the simultaneous synthesis of glucose from pyruvate in the liver (gluconeogenesis) and partial oxidation of glucose to pyruvate (glycolysis) in the periphery, primarily in muscle. The partial oxidation of a single molecule of glucose by glycolysis yields only a fraction of the ATP that could be derived from
total oxidation to CO2 and water. However, the capacity to re-synthesize glucose, using energy derived largely from fatty acid oxidation, more than compensates for the relative inefficiency: the trade-off is expanded capacity in exchange for decreased efficiency.
Ways to decrease peripheral glucose utilization The capacity to derive energy from mitochondrial fatty acid -oxidation is a criti- cally important mechanism for sparing glucose. The storage efficiency of energy as triglyceride is much greater than as hepatic glycogen. Long after liver glycogen has been depleted by starvation, the body continues to draw on the triglyceride in adi- pose tissue to provide an alternative to glucose for energy production. The process decreases the need for glucose production to a minimum, sparing it for various biosynthetic processes and for use by tissues, like red blood cells, that cannot meet their energy needs any other way. Organs, like the brain, which do not derive signif- icant amounts of energy from fatty acid -oxidation within the tissue itself, oxidize ketones produced by fatty acid oxidation in the liver. The relationship between hepatic ketogenesis and peripheral ketone utilization is reviewed in Chapter 3. 123 Hepatic syndrome
Figure 4.2 The Cori cycle.
During starvation, increased secretion of epinephrine and glucagon stimulates hormone-sensitive lipase in adipose tissue to break down triglyceride into free fatty acids and glycerol. The glycerol is taken up by the liver and converted into glucose by gluconeogenesis (Figure 4.1). The fatty acids are transported in the circulation bound to albumin to tissues like liver and muscle where they are taken up, activated by esterification with coenzyme A, and transported into mitochondria, by a process dependent on availability of carnitine. In mitochondria, they undergo -oxidation with the production of energy in the form of ATP. In the liver, the principal inter- mediate in the process, acetyl-CoA, is converted to ketones (3-hydroxybutyrate and acetoacetate) for export via the circulation to tissues, such as the brain, able to regenerate acetyl-CoA and complete the oxidation of the compound to pro- duce ATP. Defects in ketone utilization are characterized by intermittent, severe ketoacidosis (Chapter 3). Free fatty acids and their coenzyme A esters are toxic. When the mobilization of fatty acids is increased, or the capacity for mitochondrial -oxidation is exceeded, for whatever reason, any excess fatty acid is converted back to triglyceride, or it is oxidized by nonmitochondrial systems, such as microsomal -oxidation and peroxisomal -oxidation (see Figure 4.5). Mitochondrial fatty acid -oxidation depends critically on the availability of adequate amounts of carnitine. Although 124 A Clinical Guide to Inherited Metabolic Diseases
Table 4.1 Causes of secondary carnitine deficiency
Decreased biosynthesis Chronic liver disease Chronic renal disease Extreme prematurity
Inadequate intake (nutritional) Prolonged TPN in premature infants Severe protein calorie malnutrition Intestinal malabsorption Vegetarian diet
Increased losses Renal tubular dysfunction Renal failure (uremia) Hemodialysis Organic acidopathies (PA, MMA, etc.) Treatment with valproic acid UCED treated with sodium benzoate
Abbreviations: TPN, total parenteral nutrition; UCED, urea cycle enzyme defects; PA, propionic acidemia; MMA, methylmalonic acidemia. Source: See Pons & De Vivo (1995).
carnitine is synthesized endogenously, and generally occurs in ample quantities in the diet, secondary deficiency is quite common (Table 4.1). No primary disorder of carnitine biosynthesis has yet been found. However, carnitine deficiency does occur as a result of genetic defects in its cellular transport. This may take the form of systemic carnitine deficiency, characterized clinically by recurrent attacks of Reye- like encephalopathy with hypoketotic hypoglycemia or as severe cardiomyopathy. Skeletal myopathy also occurs in patients with transport defects, apparently limited to the uptake of carnitine by muscle. Carnitine also provides an alternative to coenzyme A (CoASH) in the esterifica- tion of organic acid intermediates of amino acid metabolism. Exchanging the coen- zyme A of organic acyl-CoA ester with carnitine frees CoASH. CoASH is required by many processes in intermediary metabolism, particularly related to gluconeo- genesis and ammonium metabolism. In patients with inborn errors of organic acid metabolism, such as methylmalonic acidemia, acylcarnitine esters accumulate and are excreted in the urine causing secondary carnitine depletion. Within the mitochondrial matrix, fatty acyl-CoA undergoes -oxidation. The process involves four enzymic steps operating in a cycle to shorten a fatty 125 Hepatic syndrome
acyl-CoA chain by two carbons with the release of one molecule of acetyl-CoA per turn (see Figure 9.11). Several of the steps in fatty acid transport and oxidation are catalyzed by enzymes having different substrate chain-length specificities. The most important of these from the standpoint of inherited disorders of fatty acid oxidation is the first step, catalyzed by four different fatty acyl-CoA dehydrogenase enzymes: very long-chain acyl-CoA dehydrogenase (VLCAD), long-chain acyl-CoA dehydrogenase (LCAD), medium-chain acyl-CoA dehydrogenase (MCAD), and short-chain acyl-CoA dehydrogenase (SCAD). The electrons derived from the various fatty acyl-CoA dehydrogenase reactions aretransferredtoacommonelectrontransportflavoprotein(ETF)whichisoxidized in turn by a reaction catalyzed by ETF dehydrogenase. ETF dehydrogenase catalyzes the transfer of electrons to coenzyme Q, part of Complex II of the mitochon- drial electron transport chain (see Figure 9.12). Mutations affecting the amount of function of ETF or ETF dehydrogenase cause multiple acyl-CoA dehydrogenase deficiency (GA II). See also Chapters 3.
An approach to the differential diagnosis of hypoglycemia Hypoglycemia is a common nonspecific problem in severely ill neonates and young infants, regardless of the cause of the illness. Sometimes, whether the hypoglycemia is the cause, or a nonspecific result, of illness can be difficult at first to determine. Regardless of the cause, correction of hypoglycemia without delay is at least as important as making a specific diagnosis. As a rule, when it is associated with severe systemic disease, such as sepsis, it is relatively easy to control by administration of glucose at a rate at, or slightly greater than, the normal basal glucose oxidation rate (4–6 mg/kg/min in neonates and 3–5 mg/kg/min in older infants and children). Figure 4.3 shows an overview of one approach to the diagnosis of hypoglycemia, focusing primarily on that caused by inborn errors of metabolism. The presence of nonglucose reducing substances in the urine is characteristic of untreated classical galactosemia and hereditary fructose intolerance (HFI). This is simple to determine at the bedside. Testing a few drops of urine with Benedict’s reagent or with Clinitest tablets is positive in the presence of glucose, galactose, or fructose. However, dipping the same urine with Clinistix is usually negative in these conditions, indicating that the reducing substance is not glucose. Both diseases are generally associated with other prominent clinical problems. As a rule, patients with galactosemia have other evidence of hepatocellular dysfunction, and HFI is associated with marked lactic acidosis. The glycosuria in these conditions typically clears rapidly after removal of the toxic sugars from the diet. Therefore, a negative test does not eliminate the possibility of one of these disorders, particularly if the patient has been on intravenous glucose for more than a few hours. 126 A Clinical Guide to Inherited Metabolic Diseases
Figure 4.3 Approach to the differential diagnosis of hypoglycemia. Abbreviations: SGA, small for gestational age; IDM, infant of diabetic mother; HFI, hereditary fructose intolerance; AA, amino acids; OA, organic acids; FFA, free fatty acids; FAOD, fatty acid oxidation defect; hGH, human growth hormone; T4, thyroxine; GSD, glycogen storage disease; FDPase, fructose-1,6-diphosphatase.
Because hypoglycemia is a common secondary metabolic consequence of vari- ous inborn errors of amino acid and organic acid metabolism, the investigation should include analysis of urinary organic acids and plasma amino acids and ammonium.
Primary defects in glucose production The normal physiologic response to decreased glucose production is increased mitochondrial fatty acid -oxidation and the production of ketones. Accordingly, urinary tests for ketones, another bedside test, provide an indirect indication of whether hypoglycemia is the result of inadequate production or over-utilization of glucose. The hypoglycemia caused by insulin-induced over-utilization of glucose is characteristically associated with very low plasma and urine ketone concentrations (hypoketotic hypoglycemia). However, in some disorders of glucose production, 127 Hepatic syndrome
such as GSD I and PEPCK deficiency, ketogenesis is often suppressed, and plasma and urinary ketone levels, though elevated, may be inappropriately low for the degree of hypoglycemia. The history of the relationship of the hypoglycemia to feeding is often helpful here. On the one hand, hypoketotic hypoglycemia devel- oping within several minutes of feeding, particularly if it is severe, is typical of hyperinsulinism. On the other hand, patients with defects in glycogen breakdown, gluconeogenesis, or fatty acid oxidation tend to tolerate short-term fasting much better. A significant exception is GSD I, and rare cases of GSD III, in which hypo- glycemia may develop within two to three hours of feeding. GSD I may present with hypoglycemia in the newborn period. However, it is typically not difficult to control and the liver may not be particularly enlarged. In fact, a normal three-hourly feeding schedule is generally sufficient to suppress symptomatic hypoglycemia. Affected infants usually come to attention at three to five months of age when prolonging the interval between feeds, or associated intercurrent illness, precipitates an episode of severe hypoglycemia, often heralded by aseizure or coma. Some infants come to attention as a result of failure to thrive, others because of massive hepatomegaly discovered incidentally during physical examination. Occasionally, an infant with GSD I is brought to medical attention as a result of tachypnea caused by lactic acidosis. Affected children are usually pale and pasty-looking with characteristic facies, often described as ‘cherubic’ because of the doll-like appearance caused by the chubby cheeks. Truncal obesity and marked abdominal protuberance contrast with the typically thin extremities. Recurrent nosebleeds are common as a result of a secondary defect in platelet function; platelet numbers are usually normal. In addition to hypoglycemia, laboratoryexamination typically shows lactic aci- dosis, hyperuricemia, hypertriglyceridemia, and hypophosphatemia. Serial mea- surements of plasma glucose show that the tolerance of fasting is poor, often less than three hours. The hypoglycemia is characteristically unresponsive to adminis- tration of glucagon. A distinguishing feature of GSD I is a significant rise in plasma lactate in response to glucagon. The kidneys are typically enlarged, and mild renal tubular dysfunction is common, though rarely clinically significant. The basic defect in GSD I is deficiency of the production of glucose from glucose- 6-phosphate, the final common pathway for glycogenolysis and gluconeogenesis (Figure 4.1). The most common variant of the disease (type Ia) is caused by defi- ciency of the microsomal enzyme, glucose-6-phosphatase. The enzyme is only expressed in liver and kidney, and definitive diagnosis requires enzyme analysis of one or other tissue, usually liver. Liver biopsy shows massive glycogen accumulation, including glycogen within the nucleus of hepatocytes (Figure 4.4). In addition, there is marked accumulation of macrovesicular fat, but typically no fibrosis, evidence of biliary obstruction, or inflammation. Deficiency of glycose-6-phosphatase can Figure 4.4 Electron micrograph of normal liver (a) and liver in glycogen storage disease (b). Figure a, shows normal liver. Portions of several normal hepatocytes surrounding a sinu- soidal blood space are shown. The black deposits (arrows) in the cytoplasm of liver cells are glycogen aggregates. Mitochondria (m), peroxisomes (p) and rough endoplasmic reticulum (rer) can also be seen. Note the endothelial cell processes (∗) and fenestra (arrowheads). The bar represents 1 m. Figure b, shows liver from a patient with glycogen storage disease, type Ia. Massive stores of electron dense glycogen particles (G) occupy the cytoplasm and displace mitochondria and other organelles (∗)tothe periphery of the cells. Glycogen can also be seen in the nucleus (N). The bar represents 1 m. (Courtesy of Dr. M. J. Phillips.) 129 Hepatic syndrome
often be demonstrated histochemically. However, the diagnosis should generally be confirmed by specific enzyme analysis on fresh liver obtained by biopsy. The non-type Ia variants of GSD I are caused by deficiency in the microso- mal transport of glucose-6-phosphate (type Ib), phosphate (type Ic), or glucose (type Id). Types Ib and Ic are clinically indistinguishable from type Ia. However they are also associated with persistent neutropenia, and affected children typically have histories of recurrent pyogenic infections and pyorrhea. Treatment of all types of GSD I is aimed primarily at preventing hypoglycemia by administration of frequent low-fat feeds, containing as little fructose and galactose as possible. This is supplemented by intermittent ingestion of uncooked cornstarch during the day and tube feeding with formula during the night. The neutropenia in patients with non-type Ia disease responds well to treatment with granulocyte- colony stimulating factor (G-CSF). Fasting hypoglycemia and marked hepatomegaly associated with early-onset, renal tubular dysfunction, characterized by polyuria, hypophosphatemic rickets, hyperchloremic metabolic acidosis, and severe growth retardation, is typical of Fanconi-Bickel syndrome. This condition is caused by mutations in the GLUT2 gene, coding for the liver-type glucose transporter. Hepatic glucose-6-phosphatase activity is normal. The combination of hypoglycemia, marked hepatomegaly, and lactic acidosis is also characteristic of other defects of gluconeogenesis, such as hereditary fructose intolerance, fructose-1,6-diphosphatase deficiency, PEPCK deficiency, and some- times pyruvate carboxylase (PC) deficiency. In patients with hereditary fructose intolerance (HFI), the development of symp- toms is clearly related to the ingestion of fructose or sucrose, often presenting with intractable vomiting, sometimes severe enough to suggest pyloric obstruction. Fructose ingestion often precipitates symptomatic hypoglycemia. More prolonged exposure results in failure to thrive, chronic irritability, hepatomegaly, abdomi- nal distension, edema, and jaundice. Milder variants of the disease are common. Affected patients may complain of nothing more than sugar intolerance (bloating, abdominal discomfort, diarrhea). In addition to hypoglycemia, marked lactic acidosis, hyperuricemia, and hypophosphatemia, affected patients have evidence of hepatocellular dysfunc- tion (elevated aminotransferases, increased plasma methionine and tyrosine lev- els, prolonged prothrombin and partial thromboplastin times, hypoalbuminemia, hyperbilirubinemia), and renal tubular dysfunction (hyperchloremic metabolic acidosis, generalized amino aciduria). The diagnosis is confirmed by demonstrat- ing deficiency of aldolase B (fructose-1,6-bisphosphate aldolase) in fresh liver with fructose-1-phosphate and fructose-1,6-bisphosphate as substrates. Activities with both substrates are typically markedly decreased, although the effect with 130 A Clinical Guide to Inherited Metabolic Diseases
fructose-1-phosphate as substrate is more pronounced. Fructose tolerance tests in patients with HFI are dangerous and should only be conducted under carefully controlled circumstances in patients who are in good general condition. Mutation analysis is often helpful, though failure to demonstrate a mutation does not rule out thedisease,especiallyifanalysisfocuseson thesmallnumberofcommonmutations. HFI may present clinically indistinguishable from congenital disorder of glycosyla- tion syndrome, type Ib, caused by deficiency of phosphomannose isomerase. In fact, analysis of the glycosylation pattern of plasma transferrin by isoelectric focussing may produce evidence of hypoglycosylation that is identical to that seen in CDG Ib (see Figure 6.13). However, a few weeks on a fructose-restricted diet not only results in marked clinical improvement of infants with HFI, the isoelectric focusing pattern returns to normal. Fructose-1,6-diphosphatase deficiency may be difficult to differentiate from GSD Ia.Inboth diseases, the liver may be greatly enlarged. In fructose-1,6-diphosphatase deficiency, however, the response to glucagon is preserved. Definitive diagnosis requires measurement of the enzyme in fresh liver obtained by biopsy. Mitochon- drial PEPCK deficiency is a very rare hereditary defect in gluconeogenesis associated with severe hypoglycemia, lactic acidosis, hepatomegaly, renal tubular dysfunc- tion, hypotonia, and deteriorating liver function. Liver biopsy shows microvesicular steatosis and inflammatory changes. The diagnosis can be made by demonstrat- ing deficiency of the enzyme in fibroblasts in which the mitochondrial isozyme predominates.
Over-utilization of glucose The glucose utilization rate can be measured directly by infusions of stable isotope- labeled glucose, but this is generally impractical except in centers actively involved in research on glucose metabolism. However, glucose oxidation rates can be estimated indirectly by determining the minimum rate of glucose administration needed to maintain euglycemia. This is relatively easy in neonates who are often receiving intravenous glucose. In older infants and children, the absence of ketones in the urine or depressed plasma 3-hydroxybutyrate levels during hypoglycemia is usually astrong indication that glucose utilization is increased. Increased glucose utiliza- tion (i.e., hypoketotic hypoglycemia) occurs either as a result of hyperinsulinism, or as a result of a primary or secondary defect in fatty acid oxidation. The two sit- uations are distinguishable by measurement of plasma free fatty acid levels. One of the most powerful physiologic effects of insulin is inhibition of hormone-sensitive lipase in adipose tissue. Low free fatty acid levels during hypoglycemia are a strong indication that insulin levels are abnormally elevated. Be contrast, in patients with impaired fatty acid oxidation, free fatty acid levels are typically elevated. One way to quantitate this is to calculate the ratio of free fatty acids to 3-hydroxybutyrate 131 Hepatic syndrome
Table 4.2 Approach to hypoketotic hypoglycemia
Tolerance of fasting (in hours) Possible causes Laboratory findings
Less than 1 Hyperinsulinism Low plasma FFA levels with normal FFA/3-HOB ratio; high insulin/3-HOB ratio; high insulin/glucose ratio. 1–6 GSD type 1; other defects High plasma FFA levels with increased in gluconeogenesis FFA/3-HOB ratio; lactic acidosis. 8–24 Fatty acid oxidation High plasma FFA levels with very high defects; systemic FFA/3-HOB ratio; organic aciduria; low carnitine deficiency plasma carnitine levels
Abbreviations: FFA, free fatty acids; 3-HOB, 3-hydroxybutyrate; GSD, glycogen storage disease.
(or to 3-hydroxybutyrate + acetoacetate). Hypoketotic hypoglycemia caused by hyperinsulinism is associated with a normal ratio (<2.0), while that associated with fatty acid oxidation defects is typically elevated (>3.0). In disorders of gluco- neogenesis, including GSD I, the ratio is also often elevated as a result of secondary inhibition of ketogenesis. However the timing of the hypoglycemia and other labo- ratory findings (Table 4.2)usually make differentiation of the conditions relatively straight forward. In the face of relative or absolute decrease in the capacity for mitochondrial fatty acid -oxidation, fatty acids are oxidized by nonmitochondrial oxidative pathways to produce medium-chain (6- to 10-carbon length) dicarboxylic acids (Figure 4.5). This occurs when increased fatty acid oxidative flux exceeds the normal capacity for mitochondrial -oxidation, or when normal mitochondrial fatty acid -oxidation is impaired. The first is typically associated with marked ketonuria and moderate medium-chain dicarboxylic aciduria. The ratio of adi- pate to 3-hydroxybutyrate in urine is generally <0.5. By contrast, patients with defects in mitochondrial fatty acid -oxidation characteristically have hypoke- totic hypoglycemia and marked medium-chain dicarboxylic aciduria owing to increased nonmitochondrial fatty acid oxidation. The adipate/3-hydroxybutyrate ration is >0.5. Therefore, a urinary adipate/3-hydroxybutyrate ratio >0.5 is sug- gestive, though not diagnostic of a mitochondrial fatty acid -oxidation defect (see Chapter 3). Inherited disturbances of fatty acid oxidation, such as systemic carnitine deficiency and MCAD deficiency, often present as acute or recurrent Reye- like syndrome: vomiting, lethargy, drowsiness, stupor, seizures, hepatomegaly, hypoglycemia, and hyperammonemia. These patients are particularly important to recognize because treatment is simple and effective. Moreover, since the metabolic 132 A Clinical Guide to Inherited Metabolic Diseases
Figure 4.5 Overview of fatty acid metabolism.
defects are hereditary, the siblings of affected children are at high risk for being similarly affected. The diagnosis of fatty acid oxidation defects can usually be confirmed by demon- strating the presence of high concentrations of C-6 to C-10 dicarboxylic acids (adipic, suberic, and sebacic acids) in the urine, the presence of characteristic acyl- carnitines in plasma, and the present of depressed free carnitine concentrations in plasma during acute metabolic decompensation. Since the organic acid abnormal- ities often disappear when the child is apparently healthy, diagnosis may be difficult if urine and blood samples are not saved from the time when the patient was acutely ill. Hypoglycemia is a prominent secondary metabolic phenomenon in all mito- chondrial fatty acid -oxidation defects. However, each of the disorders is also associated with other problems arising from primary and secondary effects of the respective enzyme or transport deficiencies (Table 4.3). These are described in other chapters dealing with the most prominent clinical aspects of various defects, such as acute encephalopathy, chronic myopathy, or cardiomyopathy. What was once called “leucine-sensitive hypoglycemia” has recently been shown in many infants to be a condition caused by mutations of the glutamate dehydro- genase gene (GLUD1)resulting in relative insensitivity of the enzyme to normal inhibition by GTP. Infants with this condition usually present in the first year of life with a history of recurrent hypoketotic hypoglycemia, elevated plasma insulin lev- els, and persistent hyperammonemia. Affected children generally show unexpected 133 Hepatic syndrome
Table 4.3 Relationship between metabolic defects and clinical manifestations of mitochondrial fatty acid -oxidation defects
Pathophysiology Clinical effects
Accumulation of intermediates of fatty acid oxidation Organic aciduria, acute encephalopathy, (substrate accumulation) hepatocellular dysfunction, cardiac arrhythmias. Inability to meet the energy needs of tissues that are Skeletal myopathy, cardiomyopathy highly dependent on fatty acid oxidation for energy (deficiency of product) Requirement for tissues to draw on glucose oxidation Hypoglycemia to meet energy needs (secondary metabolic abnormalities) Secondary carnitine depletion (resulting from Hypoglycemia, hyperammonemia, myopathy, accumulation and excretion of acylcarnitines) cardiomyopathy
tolerance of fasting. By contrast, ingestion of high-protein foods often precipitates hypoglycemic attacks. Plasma ammonium levels correlate poorly with dietary pro- tein intake, often remaining elevated despite aggressive dietary protein restriction and high carbohydrate intake. Affected infants and children often respond well to treatment with diazoxide.
Hepatocellular dysfunction
Inherited metabolic diseases presenting as acute hepatocellular dysfunction present a particularly challenging diagnostic problem. The resemblance of some of them to acquired disorders, particularly viral infections and intoxications, is so close that discrimination on clinical grounds alone is next to impossible. Furthermore, hepatocellular dysfunction, regardless of the underlying cause, is associated with secondary metabolic abnormalities that are often difficult to distinguish from the abnormalities observed in primary metabolic disorders. For example, increased concentrations of tyrosine in plasma are a common nonspecific metabolic man- ifestation of severe liver disease. Hypertyrosinemia is also typical of hepatorenal tyrosinemia. To make matters even more confusing, hepatorenal tyrosinemia com- monly presents in early infancy as severe liver failure. One way to approach this category of inborn errors of metabolism is to orga- nize them according to age of onset. Inherited metabolic diseases characterized by severe liver disease may present in early infancy, later in childhood, or in adulthood (Table 4.4). The presentation of inherited metabolic diseases with onset in the newborn period or early infancy as acute hepatocellular disease is characterized in most Table 4.4 Inherited metabolic diseases presenting as severe hepatocellular dysfunction organized according to age of onset
Disease Defect Distinguishing features Onset in the first few months of life Galactosemia GALT Severe hyperbilirubinemia; hemolytic anemia; coagulopathy Hepatorenal tyrosinemia Fumarylacetoacetate hydrolase Prominent coagulopathy; extreme elevation of AFP; succinylacetone in urine LCHAD deficiency Trifunctional protein (LCHAD) ‘Hepatitis’; cardiomyopathy; dicarboxylic aciduria